diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
index 34ff86ff185..33ecd80bcb5 100644
--- a/src/cmd/go/internal/modcmd/tidy.go
+++ b/src/cmd/go/internal/modcmd/tidy.go
@@ -60,6 +60,7 @@ func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 	// request that their test dependencies be included.
 	modload.ForceUseModules = true
 	modload.RootMode = modload.NeedRoot
+	modload.DisallowWriteGoMod() // Suppress writing until we've tidied the file.
 
 	modload.LoadPackages(ctx, modload.PackageOpts{
 		Tags:                     imports.AnyTags(),
@@ -72,5 +73,7 @@ func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 
 	modload.TidyBuildList()
 	modload.TrimGoSum()
+
+	modload.AllowWriteGoMod()
 	modload.WriteGoMod()
 }