diff --git a/Runtimes/Supplemental/StringProcessing/CMakeLists.txt b/Runtimes/Supplemental/StringProcessing/CMakeLists.txt
index 709ba16918411..4dd8e4fdd35b7 100644
--- a/Runtimes/Supplemental/StringProcessing/CMakeLists.txt
+++ b/Runtimes/Supplemental/StringProcessing/CMakeLists.txt
@@ -24,6 +24,9 @@ if(NOT PROJECT_IS_TOP_LEVEL)
   message(FATAL_ERROR "Swift StringProcessing must build as a standalone project")
 endif()
 
+set(CMAKE_C_VISIBILITY_PRESET "hidden")
+set(CMAKE_VISIBILITY_INLINES_HIDDEN YES)
+
 set(CMAKE_POSITION_INDEPENDENT_CODE YES)
 
 set(${PROJECT_NAME}_SWIFTC_SOURCE_DIR