diff --git a/mlir/include/mlir-c/Diagnostics.h b/mlir/include/mlir-c/Diagnostics.h
--- a/mlir/include/mlir-c/Diagnostics.h
+++ b/mlir/include/mlir-c/Diagnostics.h
@@ -45,7 +45,8 @@
  * diagnostic completely, it is expected to return success. Otherwise, it is
  * expected to return failure to indicate that other handlers should attempt to
  * process the diagnostic. */
-typedef MlirLogicalResult (*MlirDiagnosticHandler)(MlirDiagnostic);
+typedef MlirLogicalResult (*MlirDiagnosticHandler)(MlirDiagnostic,
+                                                   void *userData);
 
 /** Prints a diagnostic using the provided callback. */
 void mlirDiagnosticPrint(MlirDiagnostic diagnostic, MlirStringCallback callback,
@@ -67,9 +68,8 @@
 /** Attaches the diagnostic handler to the context. Handlers are invoked in the
  * reverse order of attachment until one of them processes the diagnostic
  * completely. Returns an identifier that can be used to detach the handler. */
-MlirDiagnosticHandlerID
-mlirContextAttachDiagnosticHandler(MlirContext context,
-                                   MlirDiagnosticHandler handler);
+MlirDiagnosticHandlerID mlirContextAttachDiagnosticHandler(
+    MlirContext context, MlirDiagnosticHandler handler, void *userData);
 
 /** Detaches an attached diagnostic handler from the context given its
  * identifier. */
diff --git a/mlir/lib/CAPI/IR/Diagnostics.cpp b/mlir/lib/CAPI/IR/Diagnostics.cpp
--- a/mlir/lib/CAPI/IR/Diagnostics.cpp
+++ b/mlir/lib/CAPI/IR/Diagnostics.cpp
@@ -52,14 +52,13 @@
   return wrap(*std::next(unwrap(diagnostic).getNotes().begin(), pos));
 }
 
-MlirDiagnosticHandlerID
-mlirContextAttachDiagnosticHandler(MlirContext context,
-                                   MlirDiagnosticHandler handler) {
+MlirDiagnosticHandlerID mlirContextAttachDiagnosticHandler(
+    MlirContext context, MlirDiagnosticHandler handler, void *userData) {
   assert(handler && "unexpected null diagnostic handler");
   DiagnosticEngine::HandlerID id =
       unwrap(context)->getDiagEngine().registerHandler(
-          [handler](Diagnostic &diagnostic) {
-            return unwrap(handler(wrap(diagnostic)));
+          [handler, userData](Diagnostic &diagnostic) {
+            return unwrap(handler(wrap(diagnostic), userData));
           });
   return static_cast<MlirDiagnosticHandlerID>(id);
 }
diff --git a/mlir/test/CAPI/ir.c b/mlir/test/CAPI/ir.c
--- a/mlir/test/CAPI/ir.c
+++ b/mlir/test/CAPI/ir.c
@@ -1064,8 +1064,8 @@
 }
 
 // Wraps a diagnostic into additional text we can match against.
-MlirLogicalResult errorHandler(MlirDiagnostic diagnostic) {
-  fprintf(stderr, "processing diagnostic <<\n");
+MlirLogicalResult errorHandler(MlirDiagnostic diagnostic, void *userData) {
+  fprintf(stderr, "processing diagnostic (userData: %d) <<\n", (int)userData);
   mlirDiagnosticPrint(diagnostic, printToStderr, NULL);
   fprintf(stderr, "\n");
   MlirLocation loc = mlirDiagnosticGetLocation(diagnostic);
@@ -1078,7 +1078,7 @@
 void testDiagnostics() {
   MlirContext ctx = mlirContextCreate();
   MlirDiagnosticHandlerID id =
-      mlirContextAttachDiagnosticHandler(ctx, errorHandler);
+      mlirContextAttachDiagnosticHandler(ctx, errorHandler, (void *)42);
   MlirLocation loc = mlirLocationUnknownGet(ctx);
   mlirEmitError(loc, "test diagnostics");
   mlirContextDetachDiagnosticHandler(ctx, id);
@@ -1281,7 +1281,7 @@
   testDiagnostics();
   // clang-format off
   // CHECK-LABEL: @test_diagnostics
-  // CHECK: processing diagnostic <<
+  // CHECK: processing diagnostic (userData: 42) <<
   // CHECK:   test diagnostics
   // CHECK:   loc(unknown)
   // CHECK: >> end of diagnostic