diff --git a/cmd/cloud_sql_proxy/cloud_sql_proxy.go b/cmd/cloud_sql_proxy/cloud_sql_proxy.go
index f59c9d430..fb8a9c43e 100644
--- a/cmd/cloud_sql_proxy/cloud_sql_proxy.go
+++ b/cmd/cloud_sql_proxy/cloud_sql_proxy.go
@@ -101,8 +101,9 @@ dropped`,
 	)
 
 	// Settings for authentication.
-	token     = flag.String("token", "", "When set, the proxy uses this Bearer token for authorization.")
-	tokenFile = flag.String("credential_file", "",
+	token      = flag.String("token", "", "When set, the proxy uses this Bearer token for authorization.")
+	loginToken = flag.String("login_token", "", "Used in conjunction with --token and --enable_iam_login only")
+	tokenFile  = flag.String("credential_file", "",
 		`If provided, this json file will be used to retrieve Service Account
 credentials.  You may set the GOOGLE_APPLICATION_CREDENTIALS environment
 variable for the same effect.`,
@@ -381,13 +382,29 @@ func authenticatedClientFromPath(ctx context.Context, f string) (*http.Client, o
 	return oauth2.NewClient(ctx, cred.TokenSource), scoped.TokenSource, nil
 }
 
+var errLoginToken = errors.New("login_token must be used with token and enable_iam_login")
+
 func authenticatedClient(ctx context.Context) (*http.Client, oauth2.TokenSource, error) {
 	if *tokenFile != "" {
 		return authenticatedClientFromPath(ctx, *tokenFile)
 	}
+	// If login token has been set, but there is no token or
+	// enable_iam_login has not been set, error.
+	if *loginToken != "" && (*token == "" || !(*enableIAMLogin)) {
+		return nil, nil, errLoginToken
+	}
+
 	if tok := *token; tok != "" {
 		src := oauth2.StaticTokenSource(&oauth2.Token{AccessToken: tok})
-		return oauth2.NewClient(ctx, src), src, nil
+		cl := oauth2.NewClient(ctx, src)
+		if *enableIAMLogin {
+			if *loginToken == "" {
+				return nil, nil, errLoginToken
+			}
+			lts := oauth2.StaticTokenSource(&oauth2.Token{AccessToken: *loginToken})
+			return cl, lts, nil
+		}
+		return cl, src, nil
 	}
 	if f := os.Getenv("GOOGLE_APPLICATION_CREDENTIALS"); f != "" {
 		return authenticatedClientFromPath(ctx, f)
diff --git a/cmd/cloud_sql_proxy/cloud_sql_proxy_test.go b/cmd/cloud_sql_proxy/cloud_sql_proxy_test.go
index 4319379bc..f9cde6b76 100644
--- a/cmd/cloud_sql_proxy/cloud_sql_proxy_test.go
+++ b/cmd/cloud_sql_proxy/cloud_sql_proxy_test.go
@@ -18,6 +18,8 @@ import (
 	"os"
 	"strings"
 	"testing"
+
+	"golang.org/x/net/context"
 )
 
 func TestVersionStripsNewline(t *testing.T) {
@@ -31,3 +33,70 @@ func TestVersionStripsNewline(t *testing.T) {
 		t.Fatalf("want = %q, got = %q", want, got)
 	}
 }
+
+func TestAuthenticatedClient(t *testing.T) {
+	tcs := []struct {
+		desc    string
+		setup   func() func()
+		wantErr bool
+	}{
+		{
+			desc: "when just token is set",
+			setup: func() func() {
+				*token = "MYTOKEN"
+				return func() {
+					*token = ""
+				}
+			},
+			wantErr: false,
+		},
+		{
+			desc: "when just login_token is set",
+			setup: func() func() {
+				*loginToken = "MYTOKEN"
+				return func() {
+					*loginToken = ""
+				}
+			},
+			wantErr: true,
+		},
+		{
+			desc: "when token and enable_iam_login are set",
+			setup: func() func() {
+				*token = "MYTOKEN"
+				*enableIAMLogin = true
+				return func() {
+					*token = ""
+					*enableIAMLogin = false
+				}
+			},
+			wantErr: true,
+		},
+		{
+			desc: "when token, login_token, and enable_iam_login are set",
+			setup: func() func() {
+				*token = "MYTOKEN"
+				*loginToken = "MYLOGINTOKEN"
+				*enableIAMLogin = true
+				return func() {
+					*token = ""
+					*loginToken = ""
+					*enableIAMLogin = false
+				}
+			},
+			wantErr: false,
+		},
+	}
+
+	for _, tc := range tcs {
+		t.Run(tc.desc, func(t *testing.T) {
+			cleanup := tc.setup()
+			defer cleanup()
+			_, _, err := authenticatedClient(context.Background())
+			gotErr := err != nil
+			if tc.wantErr != gotErr {
+				t.Fatalf("err: want = %v, got = %v", tc.wantErr, gotErr)
+			}
+		})
+	}
+}