diff --git a/tests/Makefile.am b/tests/Makefile.am
index fba18655d..9682352d0 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -123,7 +123,7 @@ dist-hook: $(BUILT_EXTRA_DIST)
 
 if HAVE_CXX
 installed_test_programs += cxx-test
-cxx_test_SOURCES = cxx-test.C
+cxx_test_SOURCES = cxx-test.cpp
 endif
 
 if ENABLE_TIMELOOP
diff --git a/tests/cxx-test.C b/tests/cxx-test.cpp
similarity index 100%
rename from tests/cxx-test.C
rename to tests/cxx-test.cpp
diff --git a/tests/meson.build b/tests/meson.build
index 162051a15..7530571bf 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -51,7 +51,7 @@ tests = [{
   },
   'timeloop' : {},
   'cxx-test' : {
-    'source' : 'cxx-test.C',
+    'source' : 'cxx-test.cpp',
     'include_directories' : gmoduleinc,
     'dependencies' : [libgio_dep],
   },