diff --git a/swift/internal/swift_interop_hint.bzl b/swift/internal/swift_interop_hint.bzl
index bed915e4b..e9db0f51f 100644
--- a/swift/internal/swift_interop_hint.bzl
+++ b/swift/internal/swift_interop_hint.bzl
@@ -17,16 +17,30 @@
 load(":swift_common.bzl", "swift_common")
 
 def _swift_interop_hint_impl(ctx):
-    # TODO(b/194733180): For now, this rule only supports interop via an
-    # auto-derived module name or an explicit module name, but still using the
-    # auto-generated module name. Add support for manual module maps and other
-    # features, like APINotes, later.
+    # TODO(b/194733180): Take advantage of the richer API to add support for
+    # other features, like APINotes, later.
     return swift_common.create_swift_interop_info(
+        module_map = ctx.file.module_map,
         module_name = ctx.attr.module_name,
     )
 
 swift_interop_hint = rule(
     attrs = {
+        "module_map": attr.label(
+            allow_single_file = True,
+            doc = """\
+An optional custom `.modulemap` file that defines the Clang module for the
+headers in the target to which this hint is applied.
+
+If this attribute is omitted, a module map will be automatically generated based
+on the headers in the hinted target.
+
+If this attribute is provided, then `module_name` must also be provided and
+match the name of the desired top-level module in the `.modulemap` file. (A
+single `.modulemap` file may define multiple top-level modules.)
+""",
+            mandatory = False,
+        ),
         "module_name": attr.string(
             doc = """\
 The name that will be used to import the hinted module into Swift.
@@ -96,6 +110,32 @@ swift_interop_hint(
 
 When this `cc_library` is a dependency of a Swift target, a module map will be
 generated for it with the module name `CSomeLib`.
+
+#### Using a custom module map
+
+In rare cases, the automatically generated module map may not be suitable. For
+example, a Swift module may depend on a C module that defines specific
+submodules, and this is not handled by the Swift build rules. In this case, you
+can provide the module map file using the `module_map` attribute.
+
+When setting the `module_map` attribute, `module_name` must also be set to the
+name of the desired top-level module; it cannot be omitted.
+
+```build
+# //my/project/BUILD
+cc_library(
+    name = "somelib",
+    srcs = ["somelib.c"],
+    hdrs = ["somelib.h"],
+    aspect_hints = [":somelib_swift_interop"],
+)
+
+swift_interop_hint(
+    name = "somelib_swift_interop",
+    module_map = "module.modulemap",
+    module_name = "CSomeLib",
+)
+```
 """,
     implementation = _swift_interop_hint_impl,
 )
diff --git a/test/BUILD b/test/BUILD
index 38d7e1de4..4a8e807e9 100644
--- a/test/BUILD
+++ b/test/BUILD
@@ -1,6 +1,7 @@
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load(":debug_settings_tests.bzl", "debug_settings_test_suite")
 load(":generated_header_tests.bzl", "generated_header_test_suite")
+load(":interop_hints_tests.bzl", "interop_hints_test_suite")
 load(":private_deps_tests.bzl", "private_deps_test_suite")
 load(":swift_through_non_swift_tests.bzl", "swift_through_non_swift_test_suite")
 
@@ -10,6 +11,8 @@ debug_settings_test_suite()
 
 generated_header_test_suite()
 
+interop_hints_test_suite()
+
 private_deps_test_suite()
 
 swift_through_non_swift_test_suite()
diff --git a/test/fixtures/interop_hints/BUILD b/test/fixtures/interop_hints/BUILD
new file mode 100644
index 000000000..2b33cb36f
--- /dev/null
+++ b/test/fixtures/interop_hints/BUILD
@@ -0,0 +1,82 @@
+load(
+    "//swift:swift.bzl",
+    "swift_interop_hint",
+    "swift_library",
+)
+load("//test/fixtures:common.bzl", "FIXTURE_TAGS")
+
+package(
+    default_visibility = ["//test:__subpackages__"],
+)
+
+licenses(["notice"])
+
+swift_library(
+    name = "import_module_name_swift",
+    srcs = ["ImportModuleName.swift"],
+    tags = FIXTURE_TAGS,
+    deps = [":cc_lib_custom_module_name"],
+)
+
+cc_library(
+    name = "cc_lib_custom_module_name",
+    hdrs = [
+        "header1.h",
+        "header2.h",
+    ],
+    aspect_hints = [":cc_lib_custom_module_name_hint"],
+    tags = FIXTURE_TAGS,
+)
+
+swift_interop_hint(
+    name = "cc_lib_custom_module_name_hint",
+    module_name = "ModuleName",
+    tags = FIXTURE_TAGS,
+)
+
+swift_library(
+    name = "import_submodule_swift",
+    srcs = ["ImportSubmodule.swift"],
+    tags = FIXTURE_TAGS,
+    deps = [":cc_lib_submodule"],
+)
+
+cc_library(
+    name = "cc_lib_submodule",
+    hdrs = [
+        "header1.h",
+        "header2.h",
+    ],
+    aspect_hints = [":cc_lib_submodule_hint"],
+    tags = FIXTURE_TAGS,
+)
+
+swift_interop_hint(
+    name = "cc_lib_submodule_hint",
+    module_map = "module.modulemap",
+    module_name = "TopModule",
+    tags = FIXTURE_TAGS,
+)
+
+swift_library(
+    name = "invalid_swift",
+    srcs = ["ImportSubmodule.swift"],
+    tags = FIXTURE_TAGS,
+    deps = [":cc_lib_invalid"],
+)
+
+cc_library(
+    name = "cc_lib_invalid",
+    hdrs = [
+        "header1.h",
+        "header2.h",
+    ],
+    aspect_hints = [":cc_lib_invalid_hint"],
+    tags = FIXTURE_TAGS,
+)
+
+swift_interop_hint(
+    name = "cc_lib_invalid_hint",
+    module_map = "module.modulemap",
+    tags = FIXTURE_TAGS,
+)
diff --git a/test/fixtures/interop_hints/ImportModuleName.swift b/test/fixtures/interop_hints/ImportModuleName.swift
new file mode 100644
index 000000000..f5e291cdd
--- /dev/null
+++ b/test/fixtures/interop_hints/ImportModuleName.swift
@@ -0,0 +1 @@
+import ModuleName
diff --git a/test/fixtures/interop_hints/ImportSubmodule.swift b/test/fixtures/interop_hints/ImportSubmodule.swift
new file mode 100644
index 000000000..856f8cbb2
--- /dev/null
+++ b/test/fixtures/interop_hints/ImportSubmodule.swift
@@ -0,0 +1 @@
+import TopModule.Submodule
diff --git a/test/fixtures/interop_hints/header1.h b/test/fixtures/interop_hints/header1.h
new file mode 100644
index 000000000..bd9ec079d
--- /dev/null
+++ b/test/fixtures/interop_hints/header1.h
@@ -0,0 +1 @@
+// Intentionally empty.
diff --git a/test/fixtures/interop_hints/header2.h b/test/fixtures/interop_hints/header2.h
new file mode 100644
index 000000000..bd9ec079d
--- /dev/null
+++ b/test/fixtures/interop_hints/header2.h
@@ -0,0 +1 @@
+// Intentionally empty.
diff --git a/test/fixtures/interop_hints/module.modulemap b/test/fixtures/interop_hints/module.modulemap
new file mode 100644
index 000000000..5338c14d8
--- /dev/null
+++ b/test/fixtures/interop_hints/module.modulemap
@@ -0,0 +1,9 @@
+module TopModule {
+  header "header1.h"
+  export *
+
+  module Submodule {
+    header "header2.h"
+    export *
+  }
+}
diff --git a/test/interop_hints_tests.bzl b/test/interop_hints_tests.bzl
new file mode 100644
index 000000000..31d6d5633
--- /dev/null
+++ b/test/interop_hints_tests.bzl
@@ -0,0 +1,69 @@
+# Copyright 2021 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for `swift_interop_hint`."""
+
+load(
+    "@build_bazel_rules_swift//test/rules:analysis_failure_test.bzl",
+    "analysis_failure_test",
+)
+load(
+    "@build_bazel_rules_swift//test/rules:provider_test.bzl",
+    "provider_test",
+)
+
+def interop_hints_test_suite():
+    """Test suite for `swift_interop_hint`."""
+    name = "interop_hints"
+
+    # Verify that a hint with only a custom module name causes the `cc_library`
+    # to propagate a `SwiftInfo` info with the expected auto-generated module
+    # map.
+    provider_test(
+        name = "{}_hint_with_custom_module_name_builds".format(name),
+        expected_files = [
+            "test/fixtures/interop_hints/cc_lib_custom_module_name.swift.modulemap",
+        ],
+        field = "transitive_modules.clang.module_map!",
+        provider = "SwiftInfo",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/interop_hints:import_module_name_swift",
+    )
+
+    # Verify that a hint with a custom module map file causes the `cc_library`
+    # to propagate a `SwiftInfo` info with that file.
+    provider_test(
+        name = "{}_hint_with_custom_module_map_builds".format(name),
+        expected_files = [
+            "test/fixtures/interop_hints/module.modulemap",
+        ],
+        field = "transitive_modules.clang.module_map!",
+        provider = "SwiftInfo",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/interop_hints:import_submodule_swift",
+    )
+
+    # Verify that the build fails if a hint provides `module_map` without
+    # `module_name`.
+    analysis_failure_test(
+        name = "{}_fails_when_module_map_provided_without_module_name".format(name),
+        expected_message = "'module_name' must be specified when 'module_map' is specified.",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/interop_hints:invalid_swift",
+    )
+
+    native.test_suite(
+        name = name,
+        tags = [name],
+    )