diff --git a/clientcredentials/clientcredentials.go b/clientcredentials/clientcredentials.go
index 2459d069f..6da071516 100644
--- a/clientcredentials/clientcredentials.go
+++ b/clientcredentials/clientcredentials.go
@@ -24,6 +24,10 @@ import (
 	"golang.org/x/oauth2/internal"
 )
 
+const (
+	ClientJWTAssertionType = "urn:ietf:params:oauth:client-assertion-type:jwt-bearer"
+)
+
 // Config describes a 2-legged OAuth2 flow, with both the
 // client application information and the server's endpoint URLs.
 type Config struct {
@@ -33,6 +37,9 @@ type Config struct {
 	// ClientSecret is the application's secret.
 	ClientSecret string
 
+	// ClientAssertionFn is a function to generate a client assertion value.
+	ClientAssertionFn func(ctx context.Context) (string, error)
+
 	// TokenURL is the resource server's token endpoint
 	// URL. This is a constant specific to each server.
 	TokenURL string
@@ -107,6 +114,16 @@ func (c *tokenSource) Token() (*oauth2.Token, error) {
 		v[k] = p
 	}
 
+	if c.conf.ClientAssertionFn != nil {
+		clientAssertion, err := c.conf.ClientAssertionFn(c.ctx)
+		if err != nil {
+			return nil, err
+		}
+
+		v.Set("client_assertion", clientAssertion)
+		v.Set("client_assertion_type", ClientJWTAssertionType)
+	}
+
 	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, v, internal.AuthStyle(c.conf.AuthStyle), c.conf.authStyleCache.Get())
 	if err != nil {
 		if rErr, ok := err.(*internal.RetrieveError); ok {
diff --git a/clientcredentials/clientcredentials_test.go b/clientcredentials/clientcredentials_test.go
index 078e75ec7..e89c2325f 100644
--- a/clientcredentials/clientcredentials_test.go
+++ b/clientcredentials/clientcredentials_test.go
@@ -12,16 +12,26 @@ import (
 	"net/http/httptest"
 	"net/url"
 	"testing"
+
+	"golang.org/x/oauth2"
 )
 
-func newConf(serverURL string) *Config {
-	return &Config{
+func newConf(serverURL string, assertion bool) *Config {
+	conf := &Config{
 		ClientID:       "CLIENT_ID",
-		ClientSecret:   "CLIENT_SECRET",
 		Scopes:         []string{"scope1", "scope2"},
 		TokenURL:       serverURL + "/token",
 		EndpointParams: url.Values{"audience": {"audience1"}},
+		AuthStyle:      oauth2.AuthStyleInParams,
+	}
+	if assertion {
+		conf.ClientAssertionFn = func(ctx context.Context) (string, error) {
+			return "CLIENT_ASSERTION", nil
+		}
+	} else {
+		conf.ClientSecret = "CLIENT_SECRET"
 	}
+	return conf
 }
 
 type mockTransport struct {
@@ -69,45 +79,70 @@ func TestTokenSourceGrantTypeOverride(t *testing.T) {
 	}
 }
 
+func assert(t *testing.T, want, got string) {
+	t.Helper()
+	if got != want {
+		t.Errorf("got %q; want %q", got, want)
+	}
+}
+
 func TestTokenRequest(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.String() != "/token" {
 			t.Errorf("authenticate client request URL = %q; want %q", r.URL, "/token")
 		}
-		headerAuth := r.Header.Get("Authorization")
-		if headerAuth != "Basic Q0xJRU5UX0lEOkNMSUVOVF9TRUNSRVQ=" {
-			t.Errorf("Unexpected authorization header, %v is found.", headerAuth)
-		}
+
 		if got, want := r.Header.Get("Content-Type"), "application/x-www-form-urlencoded"; got != want {
 			t.Errorf("Content-Type header = %q; want %q", got, want)
 		}
-		body, err := ioutil.ReadAll(r.Body)
-		if err != nil {
-			r.Body.Close()
-		}
-		if err != nil {
-			t.Errorf("failed reading request body: %s.", err)
-		}
-		if string(body) != "audience=audience1&grant_type=client_credentials&scope=scope1+scope2" {
-			t.Errorf("payload = %q; want %q", string(body), "grant_type=client_credentials&scope=scope1+scope2")
+
+		assert(t, "audience1", r.FormValue("audience"))
+		assert(t, "CLIENT_ID", r.FormValue("client_id"))
+		assert(t, "client_credentials", r.FormValue("grant_type"))
+		assert(t, "scope1 scope2", r.FormValue("scope"))
+		if r.FormValue("client_secret") != "" {
+			assert(t, "CLIENT_SECRET", r.FormValue("client_secret"))
+		} else {
+			assert(t, "CLIENT_ASSERTION", r.FormValue("client_assertion"))
+			assert(t, "urn:ietf:params:oauth:client-assertion-type:jwt-bearer", r.FormValue("client_assertion_type"))
 		}
 		w.Header().Set("Content-Type", "application/x-www-form-urlencoded")
 		w.Write([]byte("access_token=90d64460d14870c08c81352a05dedd3465940a7c&token_type=bearer"))
 	}))
 	defer ts.Close()
-	conf := newConf(ts.URL)
-	tok, err := conf.Token(context.Background())
-	if err != nil {
-		t.Error(err)
-	}
-	if !tok.Valid() {
-		t.Fatalf("token invalid. got: %#v", tok)
+
+	type testCase struct {
+		name string
+		conf *Config
 	}
-	if tok.AccessToken != "90d64460d14870c08c81352a05dedd3465940a7c" {
-		t.Errorf("Access token = %q; want %q", tok.AccessToken, "90d64460d14870c08c81352a05dedd3465940a7c")
+
+	tests := []testCase{
+		{
+			name: "client id and client_secret",
+			conf: newConf(ts.URL, false),
+		},
+		{
+			name: "client id and client_assertion",
+			conf: newConf(ts.URL, true),
+		},
 	}
-	if tok.TokenType != "bearer" {
-		t.Errorf("token type = %q; want %q", tok.TokenType, "bearer")
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			tok, err := tc.conf.Token(context.Background())
+			if err != nil {
+				t.Error(err)
+			}
+			if !tok.Valid() {
+				t.Fatalf("token invalid. got: %#v", tok)
+			}
+			if tok.AccessToken != "90d64460d14870c08c81352a05dedd3465940a7c" {
+				t.Errorf("Access token = %q; want %q", tok.AccessToken, "90d64460d14870c08c81352a05dedd3465940a7c")
+			}
+			if tok.TokenType != "bearer" {
+				t.Errorf("token type = %q; want %q", tok.TokenType, "bearer")
+			}
+		})
 	}
 }
 
@@ -132,7 +167,7 @@ func TestTokenRefreshRequest(t *testing.T) {
 		io.WriteString(w, `{"access_token": "foo", "refresh_token": "bar"}`)
 	}))
 	defer ts.Close()
-	conf := newConf(ts.URL)
+	conf := newConf(ts.URL, false)
 	c := conf.Client(context.Background())
 	c.Get(ts.URL + "/somethingelse")
 }