diff --git a/cmd/steamimmich.go b/cmd/steamimmich.go
index 206a0de..a954435 100644
--- a/cmd/steamimmich.go
+++ b/cmd/steamimmich.go
@@ -1,7 +1,10 @@
 package main
 
 import (
+	"log"
 	"os"
+	"path/filepath"
+	"runtime"
 
 	steamimmich "git.kapelle.org/niklas/steam-immich/internal"
 	"github.com/alexflint/go-arg"
@@ -19,11 +22,70 @@ func main() {
 	var args args
 	arg.MustParse(&args)
 
+	var steamUserdataDir = ""
+
+	if args.UserdataDir != "" {
+		steamUserdataDir = os.ExpandEnv(args.UserdataDir)
+	} else {
+		steamUserdataDir = findUserdataDir()
+	}
+
+	if steamUserdataDir == "" {
+		log.Fatal("Can not find steam userdata dir. Please set with --steam-userdata-dir")
+	}
+
 	steamimmich.Run(steamimmich.Config{
 		APIKey:      args.APIKey,
 		BaseURL:     args.BaseURL,
-		UserdataDir: os.ExpandEnv(args.UserdataDir),
+		UserdataDir: steamUserdataDir,
 		DeiveID:     args.DeviceID,
 		Album:       args.Album,
 	})
 }
+
+func findUserdataDir() string {
+	var candidates []string
+
+	switch runtime.GOOS {
+	case "windows":
+		// Common Steam install paths on Windows
+		localAppData := os.Getenv("LOCALAPPDATA")
+		programFiles := os.Getenv("ProgramFiles(x86)")
+		if programFiles == "" {
+			programFiles = os.Getenv("ProgramFiles")
+		}
+
+		candidates = []string{
+			filepath.Join(programFiles, "Steam"),
+			filepath.Join(localAppData, "Steam"),
+		}
+
+	case "linux":
+		home := os.Getenv("HOME")
+
+		candidates = []string{
+			filepath.Join(home, ".steam", "steam"),                                         // standard
+			filepath.Join(home, ".local", "share", "Steam"),                                // alternate
+			filepath.Join(home, ".var", "app", "com.valvesoftware.Steam", "data", "Steam"), // Flatpak
+		}
+	case "darwin":
+		// Common Steam install path on macOS
+		home := os.Getenv("HOME")
+		candidates = []string{
+			filepath.Join(home, "Library", "Application Support", "Steam"),
+		}
+
+	default:
+		return ""
+	}
+
+	for _, base := range candidates {
+		userdata := filepath.Join(base, "userdata")
+		info, err := os.Stat(userdata)
+		if err == nil && info.IsDir() {
+			return userdata
+		}
+	}
+
+	return ""
+}