diff --git a/toolchain/check/cpp/export.cpp b/toolchain/check/cpp/export.cpp
index bf8d85c013da..cffce8ade0ad 100644
--- a/toolchain/check/cpp/export.cpp
+++ b/toolchain/check/cpp/export.cpp
@@ -487,18 +487,32 @@ static auto CreateCppFieldDecl(Context& context,
     return nullptr;
   }
 
+  auto clang_loc = GetCppLocation(context, SemIR::LocId(field_inst_id));
+  bool invalid = false;
+
+  // The field is required to have a complete type in the Clang AST, not just in
+  // SemIR.
+  if (context.clang_sema().RequireCompleteSizedType(
+          clang_loc, cpp_type, clang::diag::err_field_incomplete_or_sizeless)) {
+    // Follow Clang in marking the class as invalid if it contains a field with
+    // an incomplete type. In Clang this prevents record layout; it's unclear if
+    // it's necessary for us.
+    record_decl->setInvalidDecl();
+    invalid = true;
+  }
+
   // Get the field's C++ identifier.
   auto* identifier_info = GetClangIdentifierInfo(context, field_decl.name_id);
   CARBON_CHECK(identifier_info, "field with non-identifier name {0}",
                field_decl.name_id);
 
   // Create the `clang::FieldDecl`.
-  auto clang_loc = GetCppLocation(context, SemIR::LocId(field_inst_id));
   auto* cpp_field_decl = clang::FieldDecl::Create(
       context.ast_context(), record_decl, /*StartLoc=*/clang_loc,
       /*IdLoc=*/clang_loc, identifier_info, cpp_type, /*TInfo=*/nullptr,
       /*BW=*/nullptr,
       /*Mutable=*/true, clang::ICIS_NoInit);
+  cpp_field_decl->setInvalidDecl(invalid);
 
   SetCppClassMemberAccess(class_scope, field_decl.name_id, cpp_field_decl);
 
diff --git a/toolchain/check/testdata/interop/cpp/class/export/field.carbon b/toolchain/check/testdata/interop/cpp/class/export/field.carbon
index 449bd7f92696..c584567e1746 100644
--- a/toolchain/check/testdata/interop/cpp/class/export/field.carbon
+++ b/toolchain/check/testdata/interop/cpp/class/export/field.carbon
@@ -48,6 +48,24 @@ void F() {
 }
 ''';
 
+// --- class_field.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+class ClassField {}
+
+class ClassWithClassField {
+  var c: ClassField;
+}
+
+inline Cpp '''
+void g() {
+  // In order for this to work, we need both class definitions to be exported to
+  // Clang.
+  Carbon::ClassWithClassField c;
+}
+''';
+
 // --- fail_nonexistent_field.carbon
 library "[[@TEST_NAME]]";
 import Cpp;