diff --git a/lldb/unittests/API/CMakeLists.txt b/lldb/unittests/API/CMakeLists.txt
index 8bdc806878239..06ac49244176c 100644
--- a/lldb/unittests/API/CMakeLists.txt
+++ b/lldb/unittests/API/CMakeLists.txt
@@ -16,6 +16,17 @@ if (CXX_SUPPORTS_DOCUMENTATION)
     PRIVATE -Wdocumentation)
 endif()
 
+# Apply -Wno-documentation-deprecated-sync while we migrate away from
+# report_fatal_error in llvm/include/llvm/Support/ErrorHandling.h
+# and llvm/include/llvm/Support/Error.h.
+# Remove this block of code when the migration is complete.
+# See https://github.com/llvm/llvm-project/issues/138914.
+check_cxx_compiler_flag("-Wno-documentation-deprecated-sync"
+                        CXX_SUPPORTS_NO_DOCUMENTATION_DEPRECATED_SYNC)
+if (CXX_SUPPORTS_NO_DOCUMENTATION_DEPRECATED_SYNC)
+  target_compile_options(APITests
+    PRIVATE -Wno-documentation-deprecated-sync)
+endif()
 
 if(Python3_RPATH)
   set_property(TARGET APITests APPEND PROPERTY BUILD_RPATH "${Python3_RPATH}")