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
@@ -40,12 +40,14 @@
 /// Opaque identifier of a diagnostic handler, useful to detach a handler.
 typedef uint64_t MlirDiagnosticHandlerID;
 
-/** Diagnostic handler type. Acceps a reference to a diagnostic, which is only
- * guaranteed to be live during the call. If the handler processed the
- * 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);
+/** Diagnostic handler type. Accepts a reference to a diagnostic, which is only
+ * guaranteed to be live during the call. The handler is passed the `userData`
+ * that was provided when the handler was attached to a context. If the handler
+ * processed the 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,
+                                                   void *userData);
 
 /// Prints a diagnostic using the provided callback.
 MLIR_CAPI_EXPORTED void mlirDiagnosticPrint(MlirDiagnostic diagnostic,
@@ -71,9 +73,15 @@
 
 /** 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. */
+ * completely. When a handler is invoked it is passed the `userData` that was
+ * provided when it was attached. If non-NULL, `deleteUserData` is called once
+ * the system no longer needs to call the handler (for instance after the
+ * handler is detached or the context is destroyed). Returns an identifier that
+ * can be used to detach the handler.
+ */
 MLIR_CAPI_EXPORTED MlirDiagnosticHandlerID mlirContextAttachDiagnosticHandler(
-    MlirContext context, MlirDiagnosticHandler handler);
+    MlirContext context, MlirDiagnosticHandler handler, void *userData,
+    void (*deleteUserData)(void *));
 
 /** 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
@@ -51,14 +51,20 @@
   return wrap(*std::next(unwrap(diagnostic).getNotes().begin(), pos));
 }
 
-MlirDiagnosticHandlerID
-mlirContextAttachDiagnosticHandler(MlirContext context,
-                                   MlirDiagnosticHandler handler) {
+static void deleteUserDataNoop(void *userData) {}
+
+MlirDiagnosticHandlerID mlirContextAttachDiagnosticHandler(
+    MlirContext context, MlirDiagnosticHandler handler, void *userData,
+    void (*deleteUserData)(void *)) {
   assert(handler && "unexpected null diagnostic handler");
+  if (deleteUserData == NULL) {
+    deleteUserData = deleteUserDataNoop;
+  }
+  std::shared_ptr<void> sharedUserData(userData, deleteUserData);
   DiagnosticEngine::HandlerID id =
       unwrap(context)->getDiagEngine().registerHandler(
-          [handler](Diagnostic &diagnostic) {
-            return unwrap(handler(wrap(diagnostic)));
+          [handler, sharedUserData](Diagnostic &diagnostic) {
+            return unwrap(handler(wrap(diagnostic), sharedUserData.get()));
           });
   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
@@ -421,7 +421,9 @@
   fprintf(stderr, "Op print with all flags: ");
   mlirOperationPrintWithFlags(operation, flags, printToStderr, NULL);
   fprintf(stderr, "\n");
-  // CHECK: Op print with all flags: %{{.*}} = "std.constant"() {elts = opaque<"", "0xDEADBEEF"> : tensor<4xi32>, value = 0 : index} : () -> index loc(unknown)
+  // CHECK: Op print with all flags: %{{.*}} = "std.constant"() {elts =
+  // opaque<"", "0xDEADBEEF"> : tensor<4xi32>, value = 0 : index} : () -> index
+  // loc(unknown)
 
   mlirOpPrintingFlagsDestroy(flags);
 }
@@ -911,14 +913,12 @@
       (uint64_t *)mlirDenseElementsAttrGetRawData(uint64Elements);
   int64_t *int64RawData =
       (int64_t *)mlirDenseElementsAttrGetRawData(int64Elements);
-  float *floatRawData =
-      (float *)mlirDenseElementsAttrGetRawData(floatElements);
+  float *floatRawData = (float *)mlirDenseElementsAttrGetRawData(floatElements);
   double *doubleRawData =
       (double *)mlirDenseElementsAttrGetRawData(doubleElements);
   if (uint32RawData[0] != 0u || uint32RawData[1] != 1u ||
-      int32RawData[0] != 0 || int32RawData[1] != 1 ||
-      uint64RawData[0] != 0u || uint64RawData[1] != 1u ||
-      int64RawData[0] != 0 || int64RawData[1] != 1 ||
+      int32RawData[0] != 0 || int32RawData[1] != 1 || uint64RawData[0] != 0u ||
+      uint64RawData[1] != 1u || int64RawData[0] != 0 || int64RawData[1] != 1 ||
       floatRawData[0] != 0.0f || floatRawData[1] != 1.0f ||
       doubleRawData[0] != 0.0 || doubleRawData[1] != 1.0)
     return 18;
@@ -1248,31 +1248,37 @@
 }
 
 // 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);
   mlirLocationPrint(loc, printToStderr, NULL);
   assert(mlirDiagnosticGetNumNotes(diagnostic) == 0);
-  fprintf(stderr, ">> end of diagnostic\n");
+  fprintf(stderr, ">> end of diagnostic (userData: %d)\n", (int)userData);
   return mlirLogicalResultSuccess();
 }
 
+// Logs when the delete user data callback is called
+static void deleteUserData(void *userData) {
+  fprintf(stderr, "deleting user data (userData: %d)\n", (int)userData);
+}
+
 void testDiagnostics() {
   MlirContext ctx = mlirContextCreate();
-  MlirDiagnosticHandlerID id =
-      mlirContextAttachDiagnosticHandler(ctx, errorHandler);
+  MlirDiagnosticHandlerID id = mlirContextAttachDiagnosticHandler(
+      ctx, errorHandler, (void *)42, deleteUserData);
   MlirLocation loc = mlirLocationUnknownGet(ctx);
   fprintf(stderr, "@test_diagnostics\n");
   mlirEmitError(loc, "test diagnostics");
   mlirContextDetachDiagnosticHandler(ctx, id);
   mlirEmitError(loc, "more test diagnostics");
   // CHECK-LABEL: @test_diagnostics
-  // CHECK: processing diagnostic <<
+  // CHECK: processing diagnostic (userData: 42) <<
   // CHECK:   test diagnostics
   // CHECK:   loc(unknown)
-  // CHECK: >> end of diagnostic
+  // CHECK: >> end of diagnostic (userData: 42)
+  // CHECK: deleting user data (userData: 42)
   // CHECK-NOT: processing diagnostic
   // CHECK:     more test diagnostics
 }