diff --git a/lib/libc/newlib/CMakeLists.txt b/lib/libc/newlib/CMakeLists.txt
index 6556a3e814ce03..2aa56b75c5fd0a 100644
--- a/lib/libc/newlib/CMakeLists.txt
+++ b/lib/libc/newlib/CMakeLists.txt
@@ -73,6 +73,9 @@ if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
 endif()
 zephyr_link_libraries(c)
 
+# newlib needs this to expose the full Zephyr C Library API
+zephyr_compile_definitions(_POSIX_C_SOURCE=200809L)
+
 if(CONFIG_NEWLIB_LIBC_NANO)
   zephyr_link_libraries(
     -specs=nano.specs