diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 7a8d826994..f5aac4b220 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -435,6 +435,29 @@ func CreateModFile(ctx context.Context, modPath string) {
 
 	modFileToBuildList()
 	WriteGoMod()
+
+	// Suggest running 'go mod tidy' unless the project is empty. Even if we
+	// imported all the correct requirements above, we're probably missing
+	// some sums, so the next build command in -mod=readonly will likely fail.
+	//
+	// We look for non-hidden .go files or subdirectories to determine whether
+	// this is an existing project. Walking the tree for packages would be more
+	// accurate, but could take much longer.
+	empty := true
+	fis, _ := ioutil.ReadDir(modRoot)
+	for _, fi := range fis {
+		name := fi.Name()
+		if strings.HasPrefix(name, ".") || strings.HasPrefix(name, "_") {
+			continue
+		}
+		if strings.HasSuffix(name, ".go") || fi.IsDir() {
+			empty = false
+			break
+		}
+	}
+	if !empty {
+		fmt.Fprintf(os.Stderr, "go: run 'go mod tidy' to add module requirements and sums\n")
+	}
 }
 
 // checkModulePathLax checks that the path meets some minimum requirements
diff --git a/src/cmd/go/testdata/script/mod_init_tidy.txt b/src/cmd/go/testdata/script/mod_init_tidy.txt
new file mode 100644
index 0000000000..6a37edd960
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_init_tidy.txt
@@ -0,0 +1,30 @@
+# 'go mod init' should not recommend 'go mod tidy' in an empty directory
+# (one that contains no non-hidden .go files or subdirectories).
+cd empty
+go mod init m
+! stderr tidy
+cd ..
+
+# 'go mod init' should recommend 'go mod tidy' if the directory has a .go file.
+cd pkginroot
+go mod init m
+stderr '^go: run ''go mod tidy'' to add module requirements and sums$'
+cd ..
+
+# 'go mod init' should recommend 'go mod tidy' if the directory has a
+# subdirectory. We don't walk the tree to see if it has .go files.
+cd subdir
+go mod init m
+stderr '^go: run ''go mod tidy'' to add module requirements and sums$'
+cd ..
+
+-- empty/empty.txt --
+Not a .go file. Still counts as an empty project.
+-- empty/.hidden/empty.go --
+File in hidden directory. Still as an empty project.
+-- empty/_hidden/empty.go --
+File in hidden directory. Still as an empty project.
+-- pkginroot/hello.go --
+package vendorimport
+-- subdir/sub/empty.txt --
+Subdirectory doesn't need to contain a package.