diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index c58abdf6a5..86423f118c 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -393,6 +393,7 @@ func runBuild(ctx context.Context, cmd *base.Command, args []string) {
 		// write all main packages to that directory.
 		// Otherwise require only a single package be built.
 		if fi, err := os.Stat(cfg.BuildO); (err == nil && fi.IsDir()) ||
+			strings.HasSuffix(cfg.BuildO, "/") ||
 			strings.HasSuffix(cfg.BuildO, string(os.PathSeparator)) {
 			if !explicitO {
 				base.Fatalf("go build: build output %q already exists and is a directory", cfg.BuildO)
diff --git a/src/cmd/go/testdata/script/build_output.txt b/src/cmd/go/testdata/script/build_output.txt
index ddf8f149bc..5068f4821e 100644
--- a/src/cmd/go/testdata/script/build_output.txt
+++ b/src/cmd/go/testdata/script/build_output.txt
@@ -25,13 +25,23 @@ rm bin
 
 ! exists bin
 go build -o bin/ x.go
-exists -exec bin/x
+exists -exec bin/x$GOEXE
 rm bin
 
+[windows] ! exists bin
+[windows] go build -o bin\x x.go
+[windows] exists -exec bin\x
+[windows] rm bin
+
+[windows] ! exists bin
+[windows] go build -o bin\ x.go
+[windows] exists -exec bin\x.exe
+[windows] rm bin
+
 ! exists bin
 mkdir bin
 go build -o bin x.go
-exists -exec bin/x
+exists -exec bin/x$GOEXE
 rm bin
 
 go build p.go