diff --git a/idtoken/idtoken.go b/idtoken/idtoken.go
index 3dce463bd61..152ac9c2007 100644
--- a/idtoken/idtoken.go
+++ b/idtoken/idtoken.go
@@ -8,12 +8,16 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"log"
 	"net/http"
+	"path/filepath"
+	"strings"
 
 	"cloud.google.com/go/compute/metadata"
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
 
+	"google.golang.org/api/impersonate"
 	"google.golang.org/api/internal"
 	"google.golang.org/api/option"
 	"google.golang.org/api/option/internaloption"
@@ -67,6 +71,7 @@ func NewClient(ctx context.Context, audience string, opts ...ClientOption) (*htt
 // provided and configured with the supplied options. The parameter audience may
 // not be empty.
 func NewTokenSource(ctx context.Context, audience string, opts ...ClientOption) (oauth2.TokenSource, error) {
+	option.WithScopes()
 	if audience == "" {
 		return nil, fmt.Errorf("idtoken: must supply a non-empty audience")
 	}
@@ -103,45 +108,72 @@ func newTokenSource(ctx context.Context, audience string, ds *internal.DialSetti
 }
 
 func tokenSourceFromBytes(ctx context.Context, data []byte, audience string, ds *internal.DialSettings) (oauth2.TokenSource, error) {
-	if err := isServiceAccount(data); err != nil {
-		return nil, err
-	}
-	cfg, err := google.JWTConfigFromJSON(data, ds.GetScopes()...)
+	allowedType, err := getAllowedType(data)
 	if err != nil {
 		return nil, err
 	}
-
-	customClaims := ds.CustomClaims
-	if customClaims == nil {
-		customClaims = make(map[string]interface{})
-	}
-	customClaims["target_audience"] = audience
-
-	cfg.PrivateClaims = customClaims
-	cfg.UseIDToken = true
-
-	ts := cfg.TokenSource(ctx)
-	tok, err := ts.Token()
-	if err != nil {
-		return nil, err
+	if allowedType == "service_account" {
+		cfg, err := google.JWTConfigFromJSON(data, ds.GetScopes()...)
+		if err != nil {
+			return nil, err
+		}
+		customClaims := ds.CustomClaims
+		if customClaims == nil {
+			customClaims = make(map[string]interface{})
+		}
+		customClaims["target_audience"] = audience
+
+		cfg.PrivateClaims = customClaims
+		cfg.UseIDToken = true
+
+		ts := cfg.TokenSource(ctx)
+		tok, err := ts.Token()
+		if err != nil {
+			return nil, err
+		}
+		return oauth2.ReuseTokenSource(tok, ts), nil
+	} else {
+		// if allowedType is "impersonated_service_account":
+		type url struct {
+			ServiceAccountImpersonationURL string `json:"service_account_impersonation_url"`
+		}
+		var accountUrl *url
+		if err := json.Unmarshal(data, &accountUrl); err != nil {
+			return nil, err
+		}
+		account := filepath.Base(accountUrl.ServiceAccountImpersonationURL)
+		account = strings.Split(account, ":")[0]
+
+		config := impersonate.IDTokenConfig{
+			Audience:        audience,
+			TargetPrincipal: account,
+			IncludeEmail:    true,
+		}
+		ts, err := impersonate.IDTokenSource(ctx, config)
+		if err != nil {
+			log.Println(err)
+		}
+		return ts, nil
 	}
-	return oauth2.ReuseTokenSource(tok, ts), nil
 }
 
-func isServiceAccount(data []byte) error {
+// isOfAllowedType returns the credentials type as a string, and an error.
+// allowed types are "service_account" and "impersonated_service_account"
+func getAllowedType(data []byte) (string, error) {
 	if len(data) == 0 {
-		return fmt.Errorf("idtoken: credential provided is 0 bytes")
+		return "", fmt.Errorf("idtoken: credential provided is 0 bytes")
 	}
 	var f struct {
 		Type string `json:"type"`
 	}
+	// if not service account return an error
 	if err := json.Unmarshal(data, &f); err != nil {
-		return err
+		return "", err
 	}
-	if f.Type != "service_account" {
-		return fmt.Errorf("idtoken: credential must be service_account, found %q", f.Type)
+	if f.Type != "service_account" && f.Type != "impersonated_service_account" {
+		return "", fmt.Errorf("idtoken: credential must be service_account or impersonated_service_account, found %q", f.Type)
 	}
-	return nil
+	return f.Type, nil
 }
 
 // WithCustomClaims optionally specifies custom private claims for an ID token.
diff --git a/idtoken/idtoken_test.go b/idtoken/idtoken_test.go
new file mode 100644
index 00000000000..367f12d8710
--- /dev/null
+++ b/idtoken/idtoken_test.go
@@ -0,0 +1,80 @@
+// Copyright 2020 Google LLC.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package idtoken
+
+import (
+	"context"
+	"reflect"
+	"testing"
+
+	"golang.org/x/oauth2"
+	"google.golang.org/api/internal"
+)
+
+var TokenSource oauth2.TokenSource
+
+func TestNewTokenSource(t *testing.T) {
+	tests := []struct {
+		name     string
+		ctx      context.Context
+		audience string
+		want     oauth2.TokenSource
+		wantErr  bool
+	}{
+		{
+			name:     "works",
+			ctx:      context.Background(),
+			audience: "https://apikeys.googleapis.com",
+			want:     TokenSource,
+			wantErr:  false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := NewTokenSource(tt.ctx, tt.audience)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("NewTokenSource() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			tok, err := got.Token()
+			if (err != nil) != tt.wantErr {
+				t.Errorf("NewTokenSource() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			_, err = Validate(tt.ctx, tok.AccessToken, tt.audience)
+			if err != nil {
+				t.Errorf("NewTokenSource() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_newTokenSource(t *testing.T) {
+	type args struct {
+		ctx      context.Context
+		audience string
+		ds       *internal.DialSettings
+	}
+	tests := []struct {
+		name    string
+		args    args
+		want    oauth2.TokenSource
+		wantErr bool
+	}{
+		// TODO: Add test cases.
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := newTokenSource(tt.args.ctx, tt.args.audience, tt.args.ds)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("newTokenSource() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("newTokenSource() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}