diff --git a/changelog/dmd.importc-pragma-importc-ignore.dd b/changelog/dmd.importc-pragma-importc-ignore.dd
new file mode 100644
index 000000000000..65ce74be82d2
--- /dev/null
+++ b/changelog/dmd.importc-pragma-importc-ignore.dd
@@ -0,0 +1,48 @@
+ImportC acquires `#pragma importc_ignore`
+
+ImportC now has `#pragma importc_ignore`, which is used to ignore kinds of
+declarations and definitions with specific identifiers.
+
+This is for situations where it is desirable for ImportC to look past
+a C declaration/definition of a symbol, and instead use a declaration/definition
+from an `__import`ed D module.
+
+---
+
+`#pragma importc_ignore` has one form:
+1. `#pragma importc_ignore(+/-<category>... : <identifier>,...)`
+
+That is, a sequence of plus-or-minus-prefixed categories, followed by a colon,
+and then a comma-separated list of identifiers.
+
+The categories recognized by `#pragma importc_ignore` are as follows:
+- `function_decl`: which ignores function declarations, e.g. `void foo(int);`.
+- `function_def`: which ignores function definitions, e.g. `void foo(int x) {}`.
+
+When a category is prefixed with a plus (`+`), declarations/definitions of that category
+will begin to be ignored if their identifier is included in the list of identifiers supplied to
+`#pragma importc_ignore`.
+When a category is prefixed with a minus (`-`), declarations/definitions of that category
+will no longer be ignored if their identifier is included in the list of identifiers supplied to
+`#pragma importc_ignore`.
+
+For example:
+``` c
+// We start ignoring function-declarations and function-definitions of `foo` and `bar`.
+#pragma importc_ignore(+function_decl +function_def : foo, bar)
+// This declaration of `foo` is ignored.
+void foo(int);
+// As is this definition of `foo`.
+void foo(int x)
+{}
+
+// We stop ignoring function-definitions of `bar`.
+#pragma importc_ignore(-function_def : bar)
+// This declaration of `bar` is ignored.
+void bar(int);
+// This definition of `bar` is not ignored.
+float bar(float x, float y)
+{
+    return x * y;
+}
+```
diff --git a/compiler/src/dmd/cparse.d b/compiler/src/dmd/cparse.d
index 7fbcd6d9bc1f..52d251436113 100644
--- a/compiler/src/dmd/cparse.d
+++ b/compiler/src/dmd/cparse.d
@@ -44,6 +44,9 @@ final class CParser(AST) : Parser!AST
         // #pragma pack stack
         Array!Identifier* records;      // identifers (or null)
         Array!structalign_t* packs;     // parallel alignment values
+
+        // state for #pragma importc_ignore
+        Ignored ignored;
     }
 
     /* C cannot be parsed without determining if an identifier is a type or a variable.
@@ -1922,15 +1925,18 @@ final class CParser(AST) : Parser!AST
                 auto s = cparseFunctionDefinition(id, dt.isTypeFunction(), specifier);
                 typedefTab.setDim(typedefTabLengthSave);
                 symbols = symbolsSave;
-                if (specifier.mod & MOD.x__stdcall)
+                if (!(cast(const(void)*) id in ignored.functionDefinitions))
                 {
-                    // If this function is __stdcall, wrap it in a LinkDeclaration so that
-                    // it's extern(Windows) when imported in D.
-                    auto decls = new AST.Dsymbols(1);
-                    (*decls)[0] = s;
-                    s = new AST.LinkDeclaration(s.loc, LINK.windows, decls);
+                    if (specifier.mod & MOD.x__stdcall)
+                    {
+                        // If this function is __stdcall, wrap it in a LinkDeclaration so that
+                        // it's extern(Windows) when imported in D.
+                        auto decls = new AST.Dsymbols(1);
+                        (*decls)[0] = s;
+                        s = new AST.LinkDeclaration(s.loc, LINK.windows, decls);
+                    }
+                    symbols.push(s);
                 }
-                symbols.push(s);
                 return;
             }
             AST.Dsymbol s = null;
@@ -2035,11 +2041,14 @@ final class CParser(AST) : Parser!AST
                         error("no initializer for function declaration");
                     if (specifier.scw & SCW.x_Thread_local)
                         error("functions cannot be `_Thread_local`"); // C11 6.7.1-4
-                    StorageClass stc = specifiersToSTC(level, specifier);
-                    stc &= ~STC.gshared;        // no gshared functions
-                    auto fd = new AST.FuncDeclaration(token.loc, Loc.initial, id, stc, dt, specifier.noreturn);
-                    specifiersToFuncDeclaration(fd, specifier);
-                    s = fd;
+                    if (!(cast(const(void)*) id in ignored.functionDeclarations))
+                    {
+                        StorageClass stc = specifiersToSTC(level, specifier);
+                        stc &= ~STC.gshared;        // no gshared functions
+                        auto fd = new AST.FuncDeclaration(token.loc, Loc.initial, id, stc, dt, specifier.noreturn);
+                        specifiersToFuncDeclaration(fd, specifier);
+                        s = fd;
+                    }
                 }
                 else
                 {
@@ -5599,6 +5608,8 @@ final class CParser(AST) : Parser!AST
         {
             if (token.ident == Id.pack)
                 pragmaPack(startloc, false);
+            else if (token.ident == Id.importc_ignore)
+                pragmaImportCIgnore(startloc, false);
             else
             {
                 nextToken();
@@ -5635,8 +5646,13 @@ final class CParser(AST) : Parser!AST
     {
         Token n;
         scan(&n);
-        if (n.value == TOK.identifier && n.ident == Id.pack)
-            return pragmaPack(loc, true);
+        if (n.value == TOK.identifier)
+        {
+            if (n.ident == Id.pack)
+                return pragmaPack(loc, true);
+            else if (n.ident == Id.importc_ignore)
+                return pragmaImportCIgnore(loc, true);
+        }
         if (n.value != TOK.endOfLine)
             skipToNextLine();
     }
@@ -5844,6 +5860,199 @@ final class CParser(AST) : Parser!AST
             skipToNextLine();
     }
 
+    private struct Ignored
+    {
+        // A hash-table with zero-sized values, and `Identifier`s casted to `const(void)*` for keys.
+        // We need only to test for the presence of an identifier, hence the zero-sized values.
+        // `Identifier` is an `extern(C++)` class, which an associative-array
+        // can't handle as keys, hence the cast to a pointer.
+        alias IdentifierSet = ubyte[0][const(void)*];
+
+        // A bitwise combination of categories of C constructs to ignore.
+        enum Category : uint
+        {
+            none = 0,
+            // These must be in the same order as the members of the anonymous struct below.
+            functionDeclarations = 1 << 0,
+            functionDefinitions = 1 << 1
+        }
+
+        union
+        {
+            IdentifierSet[2] byCategory;
+            struct
+            {
+                // These must be in the same order as the `Category` enum's members.
+                IdentifierSet functionDeclarations;
+                IdentifierSet functionDefinitions;
+            }
+        }
+
+        // Calls `action` for each category set in `categories`, supplying
+        // the corresponding `IdentifierSet` for that category.
+        void eachCategory(Action)(Category categories, scope Action action)
+        {
+            import core.bitop : bsr;
+
+            while (categories != 0)
+            {
+                const index = bsr(categories);
+                categories &= ~(1 << index);
+
+                action(byCategory[index]);
+            }
+        }
+
+        static bool categoryFromIdentifier(const Identifier id, out Category category)
+        {
+            if (id == Id.function_decl)
+            {
+                category = Ignored.Category.functionDeclarations;
+                return true;
+            }
+            if (id == Id.function_def)
+            {
+                category = Ignored.Category.functionDefinitions;
+                return true;
+            }
+
+            return false;
+        }
+    }
+
+    /*********
+     * `# pragma importc_ignore`
+     * An ImportC-specific pragma for specifying declarations and definitions that are to be ignored
+     * Scanner is on the `importc_ignore`
+     * Params:
+     *  startloc = location to use for error messages
+     *  useScan = use scan() to retrieve next token, instead of nextToken()
+     */
+    private void pragmaImportCIgnore(const ref Loc startloc, bool useScan)
+    {
+        const loc = startloc;
+        Token n;
+
+        /* Pull tokens from scan() or nextToken()
+         */
+        void scan(Token* t)
+        {
+            if (useScan)
+            {
+                Lexer.scan(t);
+            }
+            else
+            {
+                nextToken();
+                *t = token;
+            }
+        }
+
+        void unknownTokenFailure()
+        {
+            if (n.value != TOK.endOfLine)
+                skipToNextLine();
+        }
+
+        /* # pragma importc_ignore ( ...
+         */
+        scan(&n);
+        if (n.value != TOK.leftParenthesis)
+        {
+            error(loc, "left parenthesis expected to follow `#pragma importc_ignore` not `%s`", n.toChars());
+            return unknownTokenFailure();
+        }
+
+        Ignored.Category add;
+        Ignored.Category remove;
+
+        /* # pragma importc_ignore ( +/-<ignore-category>... :
+         */
+        for (;;)
+        {
+            scan(&n);
+            if (n.value == TOK.colon)
+                break;
+
+            if (n.value != TOK.add && n.value != TOK.min)
+            {
+                error(loc, "`+`, `-`, or a colon is expected to follow `#pragma importc_ignore(` not `%s`", n.toChars());
+                return unknownTokenFailure();
+            }
+
+            const prefix = n.value == TOK.add ? '+' : '-';
+
+            scan(&n);
+            if (n.value != TOK.identifier)
+            {
+                error(loc, "identifier expected to follow `#pragma importc_ignore(%c` not `%s`", prefix, n.toChars());
+                return unknownTokenFailure();
+            }
+
+            const id = n.ident;
+            Ignored.Category category;
+
+            if (!Ignored.categoryFromIdentifier(id, category))
+            {
+                error(loc, "`function_decl` or `function_def` expected to follow `#pragma importc_ignore(%c` not `%s`",
+                      prefix, id.toChars());
+                skipToNextLine();
+                return;
+            }
+
+            (prefix == '+' ? add : remove) |= category;
+            (prefix == '+' ? remove : add) &= ~category;
+        }
+
+        /* # pragma importc_ignore ( +/-<ignore-category>... : <identifier>... )
+         */
+        Array!Identifier identifiers;
+        for (;;)
+        {
+            scan(&n);
+
+            if (n.value == TOK.identifier)
+            {
+                auto id = n.ident;
+                identifiers.push(id);
+
+                scan(&n);
+                if (n.value == TOK.rightParenthesis)
+                    break;
+                else if (n.value != TOK.comma)
+                {
+                    error(loc, "comma or right parenthesis expected following `#pragma importc_ignore(... : %s` not `%s`",
+                          id.toChars(), n.toChars());
+                    return unknownTokenFailure();
+                }
+            }
+            else if (n.value == TOK.rightParenthesis)
+                break;
+            else
+            {
+                error(loc, "identifier or right parenthesis expected following `#pragma importc_ignore(... :` not `%s`",
+                      n.toChars());
+                return unknownTokenFailure();
+            }
+        }
+
+        skipToNextLine();
+
+        assert((add & remove) == 0, "There should be no overlap between add and remove.");
+
+        ignored.eachCategory(remove, (ref Ignored.IdentifierSet set)
+        {
+            foreach (id; identifiers)
+                set.remove(cast(const(void)*) id);
+        });
+
+        ignored.eachCategory(add, (ref Ignored.IdentifierSet set)
+        {
+            foreach (id; identifiers)
+                set[cast(const(void)*) id] = [];
+        });
+    }
+
     //}
 
     /******************************************************************************/
diff --git a/compiler/src/dmd/frontend.h b/compiler/src/dmd/frontend.h
index a208add26a6f..3853142cce66 100644
--- a/compiler/src/dmd/frontend.h
+++ b/compiler/src/dmd/frontend.h
@@ -8793,6 +8793,9 @@ struct Id final
     static Identifier* show;
     static Identifier* push;
     static Identifier* pop;
+    static Identifier* importc_ignore;
+    static Identifier* function_decl;
+    static Identifier* function_def;
     static Identifier* _pure;
     static Identifier* define;
     static Identifier* undef;
diff --git a/compiler/src/dmd/id.d b/compiler/src/dmd/id.d
index 6dbc60b020cc..98d7e7a7ccee 100644
--- a/compiler/src/dmd/id.d
+++ b/compiler/src/dmd/id.d
@@ -567,6 +567,9 @@ immutable Msgtable[] msgtable =
     { "show" },
     { "push" },
     { "pop" },
+    { "importc_ignore" },
+    { "function_decl" },
+    { "function_def" },
     { "_pure", "pure" },
     { "define" },
     { "undef" },
diff --git a/compiler/test/fail_compilation/importc_pragma_ignore.c b/compiler/test/fail_compilation/importc_pragma_ignore.c
new file mode 100644
index 000000000000..e1105e1ada9f
--- /dev/null
+++ b/compiler/test/fail_compilation/importc_pragma_ignore.c
@@ -0,0 +1,41 @@
+/* TEST_OUTPUT:
+---
+fail_compilation/importc_pragma_ignore.c(23): Error: left parenthesis expected to follow `#pragma importc_ignore` not `\n`
+fail_compilation/importc_pragma_ignore.c(24): Error: `+`, `-`, or a colon is expected to follow `#pragma importc_ignore(` not `\n`
+fail_compilation/importc_pragma_ignore.c(25): Error: `+`, `-`, or a colon is expected to follow `#pragma importc_ignore(` not `foo`
+fail_compilation/importc_pragma_ignore.c(26): Error: `+`, `-`, or a colon is expected to follow `#pragma importc_ignore(` not `function_decl`
+fail_compilation/importc_pragma_ignore.c(27): Error: identifier expected to follow `#pragma importc_ignore(+` not `\n`
+fail_compilation/importc_pragma_ignore.c(28): Error: identifier expected to follow `#pragma importc_ignore(-` not `\n`
+fail_compilation/importc_pragma_ignore.c(29): Error: identifier or right parenthesis expected following `#pragma importc_ignore(... :` not `\n`
+fail_compilation/importc_pragma_ignore.c(30): Error: identifier expected to follow `#pragma importc_ignore(+` not `123`
+fail_compilation/importc_pragma_ignore.c(31): Error: `function_decl` or `function_def` expected to follow `#pragma importc_ignore(+` not `not_a_category`
+fail_compilation/importc_pragma_ignore.c(32): Error: `+`, `-`, or a colon is expected to follow `#pragma importc_ignore(` not `\n`
+fail_compilation/importc_pragma_ignore.c(33): Error: `+`, `-`, or a colon is expected to follow `#pragma importc_ignore(` not `function_def`
+fail_compilation/importc_pragma_ignore.c(34): Error: `+`, `-`, or a colon is expected to follow `#pragma importc_ignore(` not `\n`
+fail_compilation/importc_pragma_ignore.c(35): Error: identifier or right parenthesis expected following `#pragma importc_ignore(... :` not `123`
+fail_compilation/importc_pragma_ignore.c(36): Error: comma or right parenthesis expected following `#pragma importc_ignore(... : foo` not `\n`
+fail_compilation/importc_pragma_ignore.c(37): Error: identifier or right parenthesis expected following `#pragma importc_ignore(... :` not `\n`
+fail_compilation/importc_pragma_ignore.c(38): Error: identifier or right parenthesis expected following `#pragma importc_ignore(... :` not `,`
+fail_compilation/importc_pragma_ignore.c(39): Error: identifier or right parenthesis expected following `#pragma importc_ignore(... :` not `\n`
+---
+*/
+
+#pragma importc_ignore
+#pragma importc_ignore(
+#pragma importc_ignore(foo
+#pragma importc_ignore(function_decl
+#pragma importc_ignore(+
+#pragma importc_ignore(-
+#pragma importc_ignore(:
+#pragma importc_ignore(+123
+#pragma importc_ignore(+not_a_category
+#pragma importc_ignore(+function_decl
+#pragma importc_ignore(+function_decl function_def
+#pragma importc_ignore(+function_decl -function_def
+#pragma importc_ignore(+function_decl -function_def : 123
+#pragma importc_ignore(+function_decl -function_def : foo
+#pragma importc_ignore(+function_decl -function_def : foo,
+#pragma importc_ignore(+function_decl -function_def : ,
+#pragma importc_ignore(+function_decl -function_def :
+void foo(void)
+{}
diff --git a/compiler/test/runnable/importc_pragma_ignore_c.c b/compiler/test/runnable/importc_pragma_ignore_c.c
new file mode 100644
index 000000000000..5ef2929be74b
--- /dev/null
+++ b/compiler/test/runnable/importc_pragma_ignore_c.c
@@ -0,0 +1,57 @@
+// EXTRA_SOURCES: imports/importc_pragma_ignore.d
+
+// For `size_t`.
+#include <stddef.h>
+
+__import importc_pragma_ignore;
+
+// A trailing comma is allowed for the identifiers.
+#pragma importc_ignore(+function_decl : foo, memset,)
+
+// The ignoring of `foo` and `memset` is still in effect after this.
+#pragma importc_ignore(+function_decl +function_def : bar)
+
+// This compiler should ignore this function declaration, and so the linker
+// shouldn't complain about `foo` being undefined when it's called in `useFoo`,
+// as the definition of `foo` from `importc_pragma_ignore` should be used.
+int foo(void);
+
+int useFoo()
+{
+    return foo();
+}
+
+typedef struct HasFoo
+{
+    int foo;
+}
+Foo;
+
+void *memset(void *);
+#pragma importc_ignore(-function_decl : memset)
+// The previous declaration of `memset` was ignored, so this redefinition
+// with a different type should succeed.
+void *memset(void *, int, size_t);
+
+int bar(int x)
+{
+	return 0;
+}
+
+int main(void)
+{
+    __check(useFoo() == 1);
+
+    // The ignoring shouldn't interfere with non-functions.
+    HasFoo hasFoo = {.foo = 7};
+    ++hasFoo.foo;
+    __check(hasFoo.foo == 8);
+
+    __check(bar(2) == 4);
+
+    unsigned char value = 0;
+    memset(&value, 111, 1);
+    __check(value == 111);
+
+    return 0;
+}
diff --git a/compiler/test/runnable/imports/importc_pragma_ignore.d b/compiler/test/runnable/imports/importc_pragma_ignore.d
new file mode 100644
index 000000000000..644632560240
--- /dev/null
+++ b/compiler/test/runnable/imports/importc_pragma_ignore.d
@@ -0,0 +1,24 @@
+module importc_pragma_ignore;
+
+int foo()
+{
+    return 1;
+}
+
+alias bar = _bar;
+
+int _bar(int x)
+{
+    return x * 2;
+}
+
+void* memset(void* destination, int value, size_t count)
+{
+    foreach (index; 0 .. count)
+    {
+        // Zero the memory so we can tell the difference between this and the actual memset.
+        (cast(ubyte*) destination)[index] = 0;
+    }
+
+    return destination;
+}