diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 9c31f9db14679..bdf476cf128a3 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -4507,18 +4507,16 @@ ASTReader::ASTReadResult ASTReader::ReadAST(StringRef FileName,
     }
   }
 
-  if (PP.getHeaderSearchInfo()
-          .getHeaderSearchOpts()
-          .ModulesValidateOncePerBuildSession) {
+  HeaderSearchOptions &HSOpts = PP.getHeaderSearchInfo().getHeaderSearchOpts();
+  if (HSOpts.ModulesValidateOncePerBuildSession) {
     // Now we are certain that the module and all modules it depends on are
-    // up to date.  Create or update timestamp files for modules that are
-    // located in the module cache (not for PCH files that could be anywhere
-    // in the filesystem).
+    // up-to-date. For implicitly-built module files, ensure the corresponding
+    // timestamp files are up-to-date in this build session.
     for (unsigned I = 0, N = Loaded.size(); I != N; ++I) {
       ImportedModule &M = Loaded[I];
-      if (M.Mod->Kind == MK_ImplicitModule) {
+      if (M.Mod->Kind == MK_ImplicitModule &&
+          M.Mod->InputFilesValidationTimestamp < HSOpts.BuildSessionTimestamp)
         updateModuleTimestamp(*M.Mod);
-      }
     }
   }