diff --git a/Grammar/python.gram b/Grammar/python.gram
index f3ef990923eec3..aaaf92e4a4e70b 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -1316,6 +1316,8 @@ invalid_parameters:
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ must be ahead of *") }
     | param_maybe_default+ '/' a='*' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expected comma between / and *") }
+    | a=NAME b=NAME {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected comma between parameters") }
 invalid_default:
     | a='=' &(')'|',') { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expected default value expression") }
 invalid_star_etc:
@@ -1344,6 +1346,8 @@ invalid_lambda_parameters:
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ must be ahead of *") }
     | lambda_param_maybe_default+ '/' a='*' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expected comma between / and *") }
+    | a=NAME b=NAME {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected comma between parameters") }
 invalid_lambda_parameters_helper:
     | a=lambda_slash_with_default { _PyPegen_singleton_seq(p, a) }
     | lambda_param_with_default+
diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index 0ee17849e28121..1d05cef9b3a4b4 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -521,6 +521,16 @@
 Traceback (most recent call last):
 SyntaxError: expected comma between / and *
 
+>>> def foo(a b):
+...    pass
+Traceback (most recent call last):
+SyntaxError: expected comma between parameters
+
+>>> def foo(a d=1):
+...    pass
+Traceback (most recent call last):
+SyntaxError: expected comma between parameters
+
 >>> def foo(a=1,d=,c):
 ...    pass
 Traceback (most recent call last):
@@ -568,6 +578,10 @@
 Traceback (most recent call last):
 SyntaxError: expected comma between / and *
 
+>>> lambda a b: None
+Traceback (most recent call last):
+SyntaxError: expected comma between parameters
+
 >>> lambda a,*b=3,c: None
 Traceback (most recent call last):
 SyntaxError: var-positional argument cannot have default value
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-05-10-20-34-00.gh-issue-133850.asdsd.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-10-20-34-00.gh-issue-133850.asdsd.rst
new file mode 100644
index 00000000000000..046aada27038a7
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-10-20-34-00.gh-issue-133850.asdsd.rst
@@ -0,0 +1 @@
+Improved error message for missing comma between parameters in function/lambda definition.
diff --git a/Parser/parser.c b/Parser/parser.c
index 509fac7df6e371..a7f1463a686993 100644
--- a/Parser/parser.c
+++ b/Parser/parser.c
@@ -22163,6 +22163,7 @@ invalid_dict_comprehension_rule(Parser *p)
 //     | param_no_default* '(' param_no_default+ ','? ')'
 //     | [(slash_no_default | slash_with_default)] param_maybe_default* '*' (',' | param_no_default) param_maybe_default* '/'
 //     | param_maybe_default+ '/' '*'
+//     | NAME NAME
 static void *
 invalid_parameters_rule(Parser *p)
 {
@@ -22373,6 +22374,33 @@ invalid_parameters_rule(Parser *p)
         D(fprintf(stderr, "%*c%s invalid_parameters[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "param_maybe_default+ '/' '*'"));
     }
+    { // NAME NAME
+        if (p->error_indicator) {
+            p->level--;
+            return NULL;
+        }
+        D(fprintf(stderr, "%*c> invalid_parameters[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "NAME NAME"));
+        expr_ty a;
+        expr_ty b;
+        if (
+            (a = _PyPegen_name_token(p))  // NAME
+            &&
+            (b = _PyPegen_name_token(p))  // NAME
+        )
+        {
+            D(fprintf(stderr, "%*c+ invalid_parameters[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "NAME NAME"));
+            _res = RAISE_SYNTAX_ERROR_KNOWN_RANGE ( a , b , "expected comma between parameters" );
+            if (_res == NULL && PyErr_Occurred()) {
+                p->error_indicator = 1;
+                p->level--;
+                return NULL;
+            }
+            goto done;
+        }
+        p->mark = _mark;
+        D(fprintf(stderr, "%*c%s invalid_parameters[%d-%d]: %s failed!\n", p->level, ' ',
+                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "NAME NAME"));
+    }
     _res = NULL;
   done:
     p->level--;
@@ -22754,6 +22782,7 @@ invalid_parameters_helper_rule(Parser *p)
 //     | lambda_param_no_default* '(' ','.lambda_param+ ','? ')'
 //     | [(lambda_slash_no_default | lambda_slash_with_default)] lambda_param_maybe_default* '*' (',' | lambda_param_no_default) lambda_param_maybe_default* '/'
 //     | lambda_param_maybe_default+ '/' '*'
+//     | NAME NAME
 static void *
 invalid_lambda_parameters_rule(Parser *p)
 {
@@ -22964,6 +22993,33 @@ invalid_lambda_parameters_rule(Parser *p)
         D(fprintf(stderr, "%*c%s invalid_lambda_parameters[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "lambda_param_maybe_default+ '/' '*'"));
     }
+    { // NAME NAME
+        if (p->error_indicator) {
+            p->level--;
+            return NULL;
+        }
+        D(fprintf(stderr, "%*c> invalid_lambda_parameters[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "NAME NAME"));
+        expr_ty a;
+        expr_ty b;
+        if (
+            (a = _PyPegen_name_token(p))  // NAME
+            &&
+            (b = _PyPegen_name_token(p))  // NAME
+        )
+        {
+            D(fprintf(stderr, "%*c+ invalid_lambda_parameters[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "NAME NAME"));
+            _res = RAISE_SYNTAX_ERROR_KNOWN_RANGE ( a , b , "expected comma between parameters" );
+            if (_res == NULL && PyErr_Occurred()) {
+                p->error_indicator = 1;
+                p->level--;
+                return NULL;
+            }
+            goto done;
+        }
+        p->mark = _mark;
+        D(fprintf(stderr, "%*c%s invalid_lambda_parameters[%d-%d]: %s failed!\n", p->level, ' ',
+                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "NAME NAME"));
+    }
     _res = NULL;
   done:
     p->level--;