diff --git a/toolchain/check/import_cpp.cpp b/toolchain/check/import_cpp.cpp
index f0b8ee526800..1e1125a22c00 100644
--- a/toolchain/check/import_cpp.cpp
+++ b/toolchain/check/import_cpp.cpp
@@ -700,8 +700,17 @@ static auto MapRecordType(Context& context, SemIR::LocId loc_id,
   // Check if the declaration is already mapped.
   SemIR::InstId record_inst_id = LookupClangDeclInstId(context, record_decl);
   if (!record_inst_id.has_value()) {
-    auto parent_inst_id =
-        AsCarbonNamespace(context, record_decl->getDeclContext());
+    clang::DeclContext* decl_context = record_decl->getDeclContext();
+    if (!clang::isa<clang::TranslationUnitDecl>(decl_context) &&
+        !clang::isa<clang::NamespaceDecl>(decl_context)) {
+      context.TODO(loc_id,
+                   "Unsupported mapping of a C++ record to a type within a "
+                   "declaration context that is not the translation unit or "
+                   "a namespace");
+      return {.inst_id = SemIR::ErrorInst::TypeInstId,
+              .type_id = SemIR::ErrorInst::TypeId};
+    }
+    auto parent_inst_id = AsCarbonNamespace(context, decl_context);
     auto parent_name_scope_id =
         context.insts().GetAs<SemIR::Namespace>(parent_inst_id).name_scope_id;
     SemIR::NameId record_name_id =
diff --git a/toolchain/check/testdata/interop/cpp/function/class.carbon b/toolchain/check/testdata/interop/cpp/function/class.carbon
index c76b7e680f5d..dbd9e7dd828e 100644
--- a/toolchain/check/testdata/interop/cpp/function/class.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/class.carbon
@@ -211,6 +211,39 @@ fn F() {
   //@dump-sem-ir-end
 }
 
+// ============================================================================
+// Defined class in class
+// ============================================================================
+
+// --- definition_in_outer_definition.h
+
+class O {
+ public:
+  class C {};
+};
+
+auto foo(O::C) -> void;
+
+// --- fail_todo_import_definition_in_outer_definition.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "definition_in_outer_definition.h";
+
+fn F() {
+  //@dump-sem-ir-begin
+  // CHECK:STDERR: fail_todo_import_definition_in_outer_definition.carbon:[[@LINE+7]]:3: error: semantics TODO: `Unsupported mapping of a C++ record to a type within a declaration context that is not the translation unit or a namespace` [SemanticsTodo]
+  // CHECK:STDERR:   Cpp.foo({});
+  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR: fail_todo_import_definition_in_outer_definition.carbon:[[@LINE+4]]:3: note: in `Cpp` name lookup for `foo` [InCppNameLookup]
+  // CHECK:STDERR:   Cpp.foo({});
+  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR:
+  Cpp.foo({});
+  var x: Cpp.O;
+  //@dump-sem-ir-end
+}
+
 // ============================================================================
 // Defined class and explicitly used
 // ============================================================================
@@ -653,6 +686,59 @@ fn F() {
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_todo_import_definition_in_outer_definition.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
+// CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %O: type = class_type @O [concrete]
+// CHECK:STDOUT:   %pattern_type.cff: type = pattern_type %O [concrete]
+// CHECK:STDOUT:   %Op.type.1b8: type = fn_type @Op.2, @Destroy.impl(%O) [concrete]
+// CHECK:STDOUT:   %Op.2df: %Op.type.1b8 = struct_value () [concrete]
+// CHECK:STDOUT:   %ptr.820: type = ptr_type %O [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Cpp: <namespace> = namespace file.%Cpp.import_cpp, [concrete] {
+// CHECK:STDOUT:     .foo = %foo.decl
+// CHECK:STDOUT:     .O = %O.decl
+// CHECK:STDOUT:     import Cpp//...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %foo.decl: %foo.type = fn_decl @foo [concrete = constants.%foo] {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %O.decl: type = class_decl @O [concrete = constants.%O] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %Cpp.ref.loc15: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %foo.ref: %foo.type = name_ref foo, imports.%foo.decl [concrete = constants.%foo]
+// CHECK:STDOUT:   %.loc15: %empty_struct_type = struct_literal ()
+// CHECK:STDOUT:   %foo.call: init %empty_tuple.type = call %foo.ref(<error>)
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %x.patt: %pattern_type.cff = binding_pattern x [concrete]
+// CHECK:STDOUT:     %x.var_patt: %pattern_type.cff = var_pattern %x.patt [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %x.var: ref %O = var %x.var_patt
+// CHECK:STDOUT:   %.loc16: type = splice_block %O.ref [concrete = constants.%O] {
+// CHECK:STDOUT:     %Cpp.ref.loc16: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:     %O.ref: type = name_ref O, imports.%O.decl [concrete = constants.%O]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %x: ref %O = bind_name x, %x.var
+// CHECK:STDOUT:   %Op.bound: <bound method> = bound_method %x.var, constants.%Op.2df
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %bound_method: <bound method> = bound_method %x.var, %Op.specific_fn
+// CHECK:STDOUT:   %addr: %ptr.820 = addr_of %x.var
+// CHECK:STDOUT:   %no_op: init %empty_tuple.type = call %bound_method(%addr)
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: --- import_definition_and_static_method_call_before.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
diff --git a/toolchain/check/testdata/interop/cpp/function/struct.carbon b/toolchain/check/testdata/interop/cpp/function/struct.carbon
index 9b0a9bde7a08..e35e2f0a8bb8 100644
--- a/toolchain/check/testdata/interop/cpp/function/struct.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/struct.carbon
@@ -211,6 +211,38 @@ fn F() {
   //@dump-sem-ir-end
 }
 
+// ============================================================================
+// Defined struct in struct
+// ============================================================================
+
+// --- definition_in_outer_definition.h
+
+struct O {
+  struct S {};
+};
+
+auto foo(O::S) -> void;
+
+// --- fail_todo_import_definition_in_outer_definition.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "definition_in_outer_definition.h";
+
+fn F() {
+  //@dump-sem-ir-begin
+  // CHECK:STDERR: fail_todo_import_definition_in_outer_definition.carbon:[[@LINE+7]]:3: error: semantics TODO: `Unsupported mapping of a C++ record to a type within a declaration context that is not the translation unit or a namespace` [SemanticsTodo]
+  // CHECK:STDERR:   Cpp.foo({});
+  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR: fail_todo_import_definition_in_outer_definition.carbon:[[@LINE+4]]:3: note: in `Cpp` name lookup for `foo` [InCppNameLookup]
+  // CHECK:STDERR:   Cpp.foo({});
+  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR:
+  Cpp.foo({});
+  var x: Cpp.O;
+  //@dump-sem-ir-end
+}
+
 // ============================================================================
 // Defined struct and explicitly used
 // ============================================================================
@@ -653,6 +685,59 @@ fn F() {
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_todo_import_definition_in_outer_definition.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
+// CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %O: type = class_type @O [concrete]
+// CHECK:STDOUT:   %pattern_type.cff: type = pattern_type %O [concrete]
+// CHECK:STDOUT:   %Op.type.1b8: type = fn_type @Op.2, @Destroy.impl(%O) [concrete]
+// CHECK:STDOUT:   %Op.2df: %Op.type.1b8 = struct_value () [concrete]
+// CHECK:STDOUT:   %ptr.820: type = ptr_type %O [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Cpp: <namespace> = namespace file.%Cpp.import_cpp, [concrete] {
+// CHECK:STDOUT:     .foo = %foo.decl
+// CHECK:STDOUT:     .O = %O.decl
+// CHECK:STDOUT:     import Cpp//...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %foo.decl: %foo.type = fn_decl @foo [concrete = constants.%foo] {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %O.decl: type = class_decl @O [concrete = constants.%O] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %Cpp.ref.loc15: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %foo.ref: %foo.type = name_ref foo, imports.%foo.decl [concrete = constants.%foo]
+// CHECK:STDOUT:   %.loc15: %empty_struct_type = struct_literal ()
+// CHECK:STDOUT:   %foo.call: init %empty_tuple.type = call %foo.ref(<error>)
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %x.patt: %pattern_type.cff = binding_pattern x [concrete]
+// CHECK:STDOUT:     %x.var_patt: %pattern_type.cff = var_pattern %x.patt [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %x.var: ref %O = var %x.var_patt
+// CHECK:STDOUT:   %.loc16: type = splice_block %O.ref [concrete = constants.%O] {
+// CHECK:STDOUT:     %Cpp.ref.loc16: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:     %O.ref: type = name_ref O, imports.%O.decl [concrete = constants.%O]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %x: ref %O = bind_name x, %x.var
+// CHECK:STDOUT:   %Op.bound: <bound method> = bound_method %x.var, constants.%Op.2df
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %bound_method: <bound method> = bound_method %x.var, %Op.specific_fn
+// CHECK:STDOUT:   %addr: %ptr.820 = addr_of %x.var
+// CHECK:STDOUT:   %no_op: init %empty_tuple.type = call %bound_method(%addr)
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: --- import_definition_and_static_method_call_before.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
diff --git a/toolchain/check/testdata/interop/cpp/function/union.carbon b/toolchain/check/testdata/interop/cpp/function/union.carbon
index 8eca37d7049d..a9680f63e687 100644
--- a/toolchain/check/testdata/interop/cpp/function/union.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/union.carbon
@@ -227,6 +227,39 @@ fn F() {
   //@dump-sem-ir-end
 }
 
+// ============================================================================
+// Defined union in union
+// ============================================================================
+
+// --- definition_in_outer_definition.h
+
+union O {
+ public:
+  union U {};
+};
+
+auto foo(O::U) -> void;
+
+// --- fail_todo_import_definition_in_outer_definition.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "definition_in_outer_definition.h";
+
+fn F() {
+  //@dump-sem-ir-begin
+  // CHECK:STDERR: fail_todo_import_definition_in_outer_definition.carbon:[[@LINE+7]]:3: error: semantics TODO: `Unsupported mapping of a C++ record to a type within a declaration context that is not the translation unit or a namespace` [SemanticsTodo]
+  // CHECK:STDERR:   Cpp.foo({});
+  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR: fail_todo_import_definition_in_outer_definition.carbon:[[@LINE+4]]:3: note: in `Cpp` name lookup for `foo` [InCppNameLookup]
+  // CHECK:STDERR:   Cpp.foo({});
+  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR:
+  Cpp.foo({});
+  var x: Cpp.O;
+  //@dump-sem-ir-end
+}
+
 // ============================================================================
 // Defined union and explicitly used
 // ============================================================================
@@ -669,6 +702,59 @@ fn F() {
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_todo_import_definition_in_outer_definition.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
+// CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %O: type = class_type @O [concrete]
+// CHECK:STDOUT:   %pattern_type.cff: type = pattern_type %O [concrete]
+// CHECK:STDOUT:   %Op.type.1b8: type = fn_type @Op.2, @Destroy.impl(%O) [concrete]
+// CHECK:STDOUT:   %Op.2df: %Op.type.1b8 = struct_value () [concrete]
+// CHECK:STDOUT:   %ptr.820: type = ptr_type %O [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Cpp: <namespace> = namespace file.%Cpp.import_cpp, [concrete] {
+// CHECK:STDOUT:     .foo = %foo.decl
+// CHECK:STDOUT:     .O = %O.decl
+// CHECK:STDOUT:     import Cpp//...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %foo.decl: %foo.type = fn_decl @foo [concrete = constants.%foo] {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %O.decl: type = class_decl @O [concrete = constants.%O] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %Cpp.ref.loc15: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %foo.ref: %foo.type = name_ref foo, imports.%foo.decl [concrete = constants.%foo]
+// CHECK:STDOUT:   %.loc15: %empty_struct_type = struct_literal ()
+// CHECK:STDOUT:   %foo.call: init %empty_tuple.type = call %foo.ref(<error>)
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %x.patt: %pattern_type.cff = binding_pattern x [concrete]
+// CHECK:STDOUT:     %x.var_patt: %pattern_type.cff = var_pattern %x.patt [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %x.var: ref %O = var %x.var_patt
+// CHECK:STDOUT:   %.loc16: type = splice_block %O.ref [concrete = constants.%O] {
+// CHECK:STDOUT:     %Cpp.ref.loc16: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:     %O.ref: type = name_ref O, imports.%O.decl [concrete = constants.%O]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %x: ref %O = bind_name x, %x.var
+// CHECK:STDOUT:   %Op.bound: <bound method> = bound_method %x.var, constants.%Op.2df
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %bound_method: <bound method> = bound_method %x.var, %Op.specific_fn
+// CHECK:STDOUT:   %addr: %ptr.820 = addr_of %x.var
+// CHECK:STDOUT:   %no_op: init %empty_tuple.type = call %bound_method(%addr)
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: --- import_definition_and_static_method_call_before.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {