diff --git a/mlir/test/lib/Dialect/Test/TestAttrDefs.td b/mlir/test/lib/Dialect/Test/TestAttrDefs.td
--- a/mlir/test/lib/Dialect/Test/TestAttrDefs.td
+++ b/mlir/test/lib/Dialect/Test/TestAttrDefs.td
@@ -291,4 +291,11 @@
 def TestArrayOfEnums : ArrayOfAttr<Test_Dialect, "ArrayOfEnums",
     "array_of_enums", "SimpleEnumAttr">;
 
+// Test custom directive as optional group anchor.
+def TestCustomAnchor : Test_Attr<"TestCustomAnchor"> {
+  let parameters = (ins "int":$a, OptionalParameter<"mlir::Optional<int>">:$b);
+  let mnemonic = "custom_anchor";
+  let assemblyFormat = "`<` $a (`>`) : (`,` ` ` custom<TrueFalse>($b)^ `>`)?";
+}
+
 #endif // TEST_ATTRDEFS
diff --git a/mlir/test/lib/Dialect/Test/TestAttributes.cpp b/mlir/test/lib/Dialect/Test/TestAttributes.cpp
--- a/mlir/test/lib/Dialect/Test/TestAttributes.cpp
+++ b/mlir/test/lib/Dialect/Test/TestAttributes.cpp
@@ -174,6 +174,23 @@
   return llvm::None;
 }
 
+//===----------------------------------------------------------------------===//
+// TestCustomAnchorAttr
+//===----------------------------------------------------------------------===//
+
+static ParseResult parseTrueFalse(AsmParser &p,
+                                  FailureOr<Optional<int>> &result) {
+  bool b;
+  if (p.parseInteger(b))
+    return failure();
+  result = Optional<int>(b);
+  return success();
+}
+
+static void printTrueFalse(AsmPrinter &p, Optional<int> result) {
+  p << (*result ? "true" : "false");
+}
+
 //===----------------------------------------------------------------------===//
 // Tablegen Generated Definitions
 //===----------------------------------------------------------------------===//
diff --git a/mlir/test/mlir-tblgen/attr-or-type-format-roundtrip.mlir b/mlir/test/mlir-tblgen/attr-or-type-format-roundtrip.mlir
--- a/mlir/test/mlir-tblgen/attr-or-type-format-roundtrip.mlir
+++ b/mlir/test/mlir-tblgen/attr-or-type-format-roundtrip.mlir
@@ -16,7 +16,11 @@
   // CHECK: #test.attr_with_type<i32, vector<4xi32>>
   attr4 = #test.attr_with_type<i32, vector<4xi32>>,
   // CHECK: #test.attr_self_type_format<5> : i32
-  attr5 = #test.attr_self_type_format<5> : i32
+  attr5 = #test.attr_self_type_format<5> : i32,
+  // CHECK: #test.custom_anchor<5>
+  attr6 = #test.custom_anchor<5>,
+  // CHECK: #test.custom_anchor<5, true>
+  attr7 = #test.custom_anchor<5, true>
 }
 
 // CHECK-LABEL: @test_roundtrip_default_parsers_struct
diff --git a/mlir/tools/mlir-tblgen/AttrOrTypeFormatGen.cpp b/mlir/tools/mlir-tblgen/AttrOrTypeFormatGen.cpp
--- a/mlir/tools/mlir-tblgen/AttrOrTypeFormatGen.cpp
+++ b/mlir/tools/mlir-tblgen/AttrOrTypeFormatGen.cpp
@@ -849,9 +849,18 @@
     guardOnAny(ctx, os, llvm::makeArrayRef(param), el->isInverted());
   } else if (auto *params = dyn_cast<ParamsDirective>(anchor)) {
     guardOnAny(ctx, os, params->getParams(), el->isInverted());
-  } else {
-    auto *strct = cast<StructDirective>(anchor);
+  } else if (auto *strct = dyn_cast<StructDirective>(anchor)) {
     guardOnAny(ctx, os, strct->getParams(), el->isInverted());
+  } else {
+    auto *custom = cast<CustomDirective>(anchor);
+    guardOnAny(ctx, os,
+               llvm::make_filter_range(
+                   llvm::map_range(custom->getArguments(),
+                                   [](FormatElement *el) {
+                                     return dyn_cast<ParameterElement>(el);
+                                   }),
+                   [](ParameterElement *param) { return !!param; }),
+               el->isInverted());
   }
   // Generate the printer for the contained elements.
   {
@@ -994,13 +1003,33 @@
         return emitError(loc, "`struct` is only allowed in an optional group "
                               "if all captured parameters are optional");
       }
+    } else if (auto *custom = dyn_cast<CustomDirective>(el)) {
+      for (FormatElement *el : custom->getArguments()) {
+        // If the custom argument is a variable, then it must be optional.
+        if (auto param = dyn_cast<ParameterElement>(el);
+            param && !param->isOptional())
+          return emitError(loc, "`custom` is only allowed in an optional group "
+                                "if all captured parameters are optional");
+      }
     }
   }
   // The anchor must be a parameter or one of the aforementioned directives.
-  if (anchor &&
-      !isa<ParameterElement, ParamsDirective, StructDirective>(anchor)) {
-    return emitError(loc,
-                     "optional group anchor must be a parameter or directive");
+  if (anchor) {
+    if (!isa<ParameterElement, ParamsDirective, StructDirective,
+             CustomDirective>(anchor)) {
+      return emitError(
+          loc, "optional group anchor must be a parameter or directive");
+    }
+    // If the anchor is a custom directive, make sure at least one of its
+    // arguments is a bound parameter.
+    if (auto custom = dyn_cast<CustomDirective>(anchor)) {
+      auto bound = llvm::find_if(custom->getArguments(), [](FormatElement *el) {
+        return isa<ParameterElement>(el);
+      });
+      if (bound == custom->getArguments().end())
+        return emitError(loc, "`custom` directive with no bound parameters "
+                              "cannot be used as optional group anchor");
+    }
   }
   return success();
 }