diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
index 4e69da567b..9dd90ee871 100644
--- a/src/cmd/go/internal/cfg/cfg.go
+++ b/src/cmd/go/internal/cfg/cfg.go
@@ -73,6 +73,11 @@ var (
 	// in module-aware mode (as opposed to GOPATH mode).
 	// It is equal to modload.Enabled, but not all packages can import modload.
 	ModulesEnabled bool
+
+	// GoModInGOPATH records whether we've found a go.mod in GOPATH/src
+	// in GO111MODULE=auto mode. In that case, we don't use modules
+	// but people might expect us to, so 'go get' warns.
+	GoModInGOPATH string
 )
 
 func init() {
diff --git a/src/cmd/go/internal/get/get.go b/src/cmd/go/internal/get/get.go
index 36aa171c7f..0789d5bb3c 100644
--- a/src/cmd/go/internal/get/get.go
+++ b/src/cmd/go/internal/get/get.go
@@ -118,6 +118,11 @@ func runGet(cmd *base.Command, args []string) {
 		// Should not happen: main.go should install the separate module-enabled get code.
 		base.Fatalf("go get: modules not implemented")
 	}
+	if cfg.GoModInGOPATH != "" {
+		// Warn about not using modules with GO111MODULE=auto when go.mod exists.
+		// To silence the warning, users can set GO111MODULE=off.
+		fmt.Fprintf(os.Stderr, "go get: warning: modules disabled by GO111MODULE=auto in GOPATH/src;\n\tignoring %s;\n\tsee 'go help modules'\n", base.ShortPath(cfg.GoModInGOPATH))
+	}
 
 	work.BuildInit()
 
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 602d33acb3..dfab6578a9 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -153,6 +153,9 @@ func Init() {
 	} else {
 		if inGOPATH && !MustUseModules {
 			// No automatic enabling in GOPATH.
+			if root, _ := FindModuleRoot(cwd, "", false); root != "" {
+				cfg.GoModInGOPATH = filepath.Join(root, "go.mod")
+			}
 			return
 		}
 		root, _ := FindModuleRoot(cwd, "", MustUseModules)
diff --git a/src/cmd/go/testdata/script/mod_get_warning.txt b/src/cmd/go/testdata/script/mod_get_warning.txt
new file mode 100644
index 0000000000..36b5434c3b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_warning.txt
@@ -0,0 +1,10 @@
+# go get in GO111MODULE=auto should warn when not using modules and go.mod exists
+
+env GO111MODULE=auto
+mkdir z
+cd z
+! go get # fails because no code in directory, not the warning
+stderr 'go get: warning: modules disabled by GO111MODULE=auto in GOPATH/src;\n\tignoring ..[/\\]go.mod;\n\tsee ''go help modules'''
+
+-- go.mod --
+module x