diff --git a/git.go b/git.go
index 20be334..2a37b7d 100644
--- a/git.go
+++ b/git.go
@@ -1,6 +1,12 @@
 package main
 
-import "os/exec"
+import (
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
 
 func SetGitConfig(user User, scope Scope, users []User, selectedUserIndex int) error {
 	cmdName := exec.Command("git", "config", scope.String(), "user.name", user.Name)
@@ -26,3 +32,23 @@ func SetGitConfig(user User, scope Scope, users []User, selectedUserIndex int) e
 
 	return nil
 }
+
+func IsUnderGitDir() error {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+
+	if _, err := os.Stat(filepath.Join(cwd, ".git")); errors.Is(err, os.ErrNotExist) {
+		return fmt.Errorf("you need to be in a git project directory to use gitsu")
+	} else if err != nil {
+		return err
+	}
+
+	cmdRevParse := exec.Command("git", "rev-parse", "--is-inside-work-tree")
+	if err := cmdRevParse.Run(); err != nil {
+		return err
+	}
+
+	return nil
+}
diff --git a/main.go b/main.go
index 5de9c84..b970e25 100644
--- a/main.go
+++ b/main.go
@@ -76,6 +76,11 @@ func run() int {
 
 	switch actionType {
 	case sel:
+		if err := IsUnderGitDir(); err != nil {
+			fmt.Fprintf(os.Stderr, "Failed to detect git project: %v\n", err)
+			return 1
+		}
+
 		if err := selectUser(); err != nil {
 			fmt.Fprintf(os.Stderr, "Failed to select user: %v\n", err)
 			return 1