diff --git a/cmake/modules/RootCTest.cmake b/cmake/modules/RootCTest.cmake
index 54338cd765a18..5db33bbb287d8 100644
--- a/cmake/modules/RootCTest.cmake
+++ b/cmake/modules/RootCTest.cmake
@@ -51,6 +51,21 @@ foreach(d ${test_list})
   endif()
 endforeach()
 
+# Building a test executable must bring lib/modules.idx and all module
+# artifacts (PCMs) up to date. Otherwise, a targeted build that changes a
+# dictionary (e.g. `cmake --build . --target testFoo`) leaves the test running
+# against a stale global module index and stale dependent PCMs, which
+# manifests as modules that fail to load or as spurious segfaults. The
+# modules_idx target depends on every ROOT module, so making the test
+# executables depend on it ensures any dictionary change is first propagated
+# through the dependent PCMs and the index is regenerated.
+if(runtime_cxxmodules AND TARGET modules_idx)
+  get_property(modules_idx_gtests GLOBAL PROPERTY ROOT_MODULES_IDX_GTESTS)
+  foreach(modules_idx_gtest ${modules_idx_gtests})
+    add_dependencies(${modules_idx_gtest} modules_idx)
+  endforeach()
+endif()
+
 # When ninja or the Microsoft generator are in use, tests that compile an executable might try
 # to rebuild the entire build tree. If multiple of these are invoked in parallel, ninja will
 # suffer from race conditions.
diff --git a/cmake/modules/RootMacros.cmake b/cmake/modules/RootMacros.cmake
index 87080dc7a5ba9..5ca11a2d2d679 100644
--- a/cmake/modules/RootMacros.cmake
+++ b/cmake/modules/RootMacros.cmake
@@ -1965,6 +1965,11 @@ function(ROOT_ADD_GTEST test_suite)
   # against. For example, tests in Core should link only against libCore. This could be tricky
   # to implement because some ROOT components create more than one library.
   ROOT_EXECUTABLE(${test_suite} ${source_files} LIBRARIES ${ARG_LIBRARIES})
+  if(runtime_cxxmodules)
+    # Register the test so that the modules_idx dependency can be attached at
+    # the end of the top-level CMakeLists, where the modules_idx target exists.
+    set_property(GLOBAL APPEND PROPERTY ROOT_MODULES_IDX_GTESTS ${test_suite})
+  endif()
   target_link_libraries(${test_suite} PRIVATE GTest::gtest GTest::gmock GTest::gtest_main GTest::gmock_main)
   if(TARGET ROOT::TestSupport)
     target_link_libraries(${test_suite} PRIVATE ROOT::TestSupport)