diff --git a/llvm/lib/MC/MCAsmStreamer.cpp b/llvm/lib/MC/MCAsmStreamer.cpp
index be8c022f39ad1..7b1a1878915bc 100644
--- a/llvm/lib/MC/MCAsmStreamer.cpp
+++ b/llvm/lib/MC/MCAsmStreamer.cpp
@@ -764,10 +764,18 @@ bool MCAsmStreamer::emitSymbolAttribute(MCSymbol *Symbol,
     OS << "\t.extern\t";
     break;
   case MCSA_Weak:           OS << MAI->getWeakDirective(); break;
-  case MCSA_WeakDefinition:
-    OS << "\t.weak_definition\t";
+  case MCSA_WeakDefinition: {
+    // AIX, use the standard weak directive (.weak) instead of
+    //'.weak_definition' because the AIX assembler does not
+    // recognize the '.weak_definition' directive.
+    const llvm::Triple &TT = getContext().getTargetTriple();
+    if (TT.isOSAIX())
+      OS << MAI->getWeakDirective();
+    else
+      OS << "\t.weak_definition\t";
     break;
-      // .weak_reference
+  }
+    // .weak_reference
   case MCSA_WeakReference:  OS << MAI->getWeakRefDirective(); break;
   case MCSA_WeakDefAutoPrivate: OS << "\t.weak_def_can_be_hidden\t"; break;
   case MCSA_Cold:
diff --git a/llvm/lib/MC/MCXCOFFStreamer.cpp b/llvm/lib/MC/MCXCOFFStreamer.cpp
index 4bf14c11068cb..cd5398dc7bc3b 100644
--- a/llvm/lib/MC/MCXCOFFStreamer.cpp
+++ b/llvm/lib/MC/MCXCOFFStreamer.cpp
@@ -72,6 +72,13 @@ bool MCXCOFFStreamer::emitSymbolAttribute(MCSymbol *Sym,
     Symbol->setStorageClass(XCOFF::C_WEAKEXT);
     Symbol->setExternal(true);
     break;
+  case llvm::MCSA_WeakDefinition:
+    // On AIX/XCOFF, a weak definition symbol should be emitted
+    // as an external weak symbol (C_WEAKEXT), since the assembler
+    // does not support '.weak_definition' directly.
+    Symbol->setStorageClass(XCOFF::C_WEAKEXT);
+    Symbol->setExternal(true);
+    break;
   case llvm::MCSA_Hidden:
     Symbol->setVisibilityType(XCOFF::SYM_V_HIDDEN);
     break;
diff --git a/llvm/test/MC/PowerPC/aix-weak-definition.s b/llvm/test/MC/PowerPC/aix-weak-definition.s
new file mode 100644
index 0000000000000..6bab51b91c63d
--- /dev/null
+++ b/llvm/test/MC/PowerPC/aix-weak-definition.s
@@ -0,0 +1,13 @@
+## Check that weak_definition symbols are emitted as weak externals.
+# Note: On AIX, .weak_definition is mapped to .weak by LLVM's backend.
+# RUN: llvm-mc -triple powerpc-ibm-aix-xcoff %s -filetype=obj -o - | \
+# RUN:   llvm-objdump --syms - | FileCheck %s
+
+        .weak_definition foo  # LLVM IR WeakDefinition → .weak on AIX
+foo:
+        blr
+
+# CHECK:      SYMBOL TABLE:
+# CHECK-NEXT: 00000000      df *DEBUG*    00000000 .file
+# CHECK-NEXT: 00000000 l       .text    00000004
+# CHECK-NEXT: 00000000 w     F .text (csect: )  00000000 foo