diff --git a/toolchain/check/testdata/interop/cpp/function_full_semir.carbon b/toolchain/check/testdata/interop/cpp/function/full_semir.carbon
similarity index 99%
rename from toolchain/check/testdata/interop/cpp/function_full_semir.carbon
rename to toolchain/check/testdata/interop/cpp/function/full_semir.carbon
index 8119822fe694..aef2430d6c7c 100644
--- a/toolchain/check/testdata/interop/cpp/function_full_semir.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/full_semir.carbon
@@ -7,9 +7,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_full_semir.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/full_semir.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_full_semir.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/full_semir.carbon
 
 // ============================================================================
 // Function with short parameter
diff --git a/toolchain/check/testdata/interop/cpp/function_param_int16.carbon b/toolchain/check/testdata/interop/cpp/function/param_int16.carbon
similarity index 99%
rename from toolchain/check/testdata/interop/cpp/function_param_int16.carbon
rename to toolchain/check/testdata/interop/cpp/function/param_int16.carbon
index 06539dafe566..f2ddf6a62986 100644
--- a/toolchain/check/testdata/interop/cpp/function_param_int16.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/param_int16.carbon
@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_param_int16.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/param_int16.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_param_int16.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/param_int16.carbon
 
 // ============================================================================
 // short
diff --git a/toolchain/check/testdata/interop/cpp/function_param_int32.carbon b/toolchain/check/testdata/interop/cpp/function/param_int32.carbon
similarity index 99%
rename from toolchain/check/testdata/interop/cpp/function_param_int32.carbon
rename to toolchain/check/testdata/interop/cpp/function/param_int32.carbon
index 7f0553435aa5..b33925bcb061 100644
--- a/toolchain/check/testdata/interop/cpp/function_param_int32.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/param_int32.carbon
@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_param_int32.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/param_int32.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_param_int32.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/param_int32.carbon
 
 // ============================================================================
 // int
diff --git a/toolchain/check/testdata/interop/cpp/function_param_unsupported.carbon b/toolchain/check/testdata/interop/cpp/function/param_unsupported.carbon
similarity index 97%
rename from toolchain/check/testdata/interop/cpp/function_param_unsupported.carbon
rename to toolchain/check/testdata/interop/cpp/function/param_unsupported.carbon
index 176c0c136ff6..3a2f6176f6ae 100644
--- a/toolchain/check/testdata/interop/cpp/function_param_unsupported.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/param_unsupported.carbon
@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_param_unsupported.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/param_unsupported.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_param_unsupported.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/param_unsupported.carbon
 
 // ============================================================================
 // Unsupported primitive type
diff --git a/toolchain/check/testdata/interop/cpp/function_return.carbon b/toolchain/check/testdata/interop/cpp/function/return.carbon
similarity index 98%
rename from toolchain/check/testdata/interop/cpp/function_return.carbon
rename to toolchain/check/testdata/interop/cpp/function/return.carbon
index 90e1b6150adf..a998ed0e6116 100644
--- a/toolchain/check/testdata/interop/cpp/function_return.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/return.carbon
@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_return.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/return.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_return.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/return.carbon
 
 // ============================================================================
 // short