diff --git a/jwt/jwt.go b/jwt/jwt.go
index 0783a94..99f3e0a 100644
--- a/jwt/jwt.go
+++ b/jwt/jwt.go
@@ -61,6 +61,11 @@ type Config struct {
 
 	// Expires optionally specifies how long the token is valid for.
 	Expires time.Duration
+
+	// Audience optionally specifies the intended audience of the
+	// request.  If empty, the value of TokenURL is used as the
+	// intended audience.
+	Audience string
 }
 
 // TokenSource returns a JWT TokenSource using the configuration
@@ -105,6 +110,9 @@ func (js jwtSource) Token() (*oauth2.Token, error) {
 	if t := js.conf.Expires; t > 0 {
 		claimSet.Exp = time.Now().Add(t).Unix()
 	}
+	if aud := js.conf.Audience; aud != "" {
+		claimSet.Aud = aud
+	}
 	h := *defaultHeader
 	h.KeyID = js.conf.PrivateKeyID
 	payload, err := jws.Encode(&h, claimSet, pk)
diff --git a/jwt/jwt_test.go b/jwt/jwt_test.go
index 1fbb9aa..9dfa3b3 100644
--- a/jwt/jwt_test.go
+++ b/jwt/jwt_test.go
@@ -191,6 +191,80 @@ func TestJWTFetch_Assertion(t *testing.T) {
 	}
 }
 
+func TestJWTFetch_AssertionPayload(t *testing.T) {
+	var assertion string
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		r.ParseForm()
+		assertion = r.Form.Get("assertion")
+
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{
+			"access_token": "90d64460d14870c08c81352a05dedd3465940a7c",
+			"scope": "user",
+			"token_type": "bearer",
+			"expires_in": 3600
+		}`))
+	}))
+	defer ts.Close()
+
+	for _, conf := range []*Config{
+		{
+			Email:        "aaa1@xxx.com",
+			PrivateKey:   dummyPrivateKey,
+			PrivateKeyID: "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
+			TokenURL:     ts.URL,
+		},
+		{
+			Email:        "aaa2@xxx.com",
+			PrivateKey:   dummyPrivateKey,
+			PrivateKeyID: "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
+			TokenURL:     ts.URL,
+			Audience:     "https://example.com",
+		},
+	} {
+		t.Run(conf.Email, func(t *testing.T) {
+			_, err := conf.TokenSource(context.Background()).Token()
+			if err != nil {
+				t.Fatalf("Failed to fetch token: %v", err)
+			}
+
+			parts := strings.Split(assertion, ".")
+			if len(parts) != 3 {
+				t.Fatalf("assertion = %q; want 3 parts", assertion)
+			}
+			gotjson, err := base64.RawURLEncoding.DecodeString(parts[1])
+			if err != nil {
+				t.Fatalf("invalid token payload; err = %v", err)
+			}
+
+			claimSet := jws.ClaimSet{}
+			if err := json.Unmarshal(gotjson, &claimSet); err != nil {
+				t.Errorf("failed to unmarshal json token payload = %q; err = %v", gotjson, err)
+			}
+
+			if got, want := claimSet.Iss, conf.Email; got != want {
+				t.Errorf("payload email = %q; want %q", got, want)
+			}
+			if got, want := claimSet.Scope, strings.Join(conf.Scopes, " "); got != want {
+				t.Errorf("payload scope = %q; want %q", got, want)
+			}
+			aud := conf.TokenURL
+			if conf.Audience != "" {
+				aud = conf.Audience
+			}
+			if got, want := claimSet.Aud, aud; got != want {
+				t.Errorf("payload audience = %q; want %q", got, want)
+			}
+			if got, want := claimSet.Sub, conf.Subject; got != want {
+				t.Errorf("payload subject = %q; want %q", got, want)
+			}
+			if got, want := claimSet.Prn, conf.Subject; got != want {
+				t.Errorf("payload prn = %q; want %q", got, want)
+			}
+		})
+	}
+}
+
 func TestTokenRetrieveError(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-type", "application/json")