diff --git a/lib/googleauth/credentials.rb b/lib/googleauth/credentials.rb
index 520371d..f49ca15 100644
--- a/lib/googleauth/credentials.rb
+++ b/lib/googleauth/credentials.rb
@@ -185,6 +185,13 @@ module Google
       #
       attr_reader :project_id
 
+      ##
+      # Identifier for a separate project used for billing/quota, if any.
+      #
+      # @return [String,nil]
+      #
+      attr_reader :quota_project_id
+
       # @private Delegate client methods to the client object.
       extend Forwardable
 
@@ -215,8 +222,6 @@ module Google
                      :token_credential_uri, :audience,
                      :scope, :issuer, :signing_key, :updater_proc
 
-      # rubocop:disable Metrics/AbcSize
-
       ##
       # Creates a new Credentials instance with the provided auth credentials, and with the default
       # values configured on the class.
@@ -236,23 +241,15 @@ module Google
       #   * +:default_connection+ - the default connection to use for the client
       #
       def initialize keyfile, options = {}
-        scope = options[:scope]
         verify_keyfile_provided! keyfile
         @project_id = options["project_id"] || options["project"]
+        @quota_project_id = options["quota_project_id"]
         if keyfile.is_a? Signet::OAuth2::Client
-          @client = keyfile
-          @project_id ||= keyfile.project_id if keyfile.respond_to? :project_id
+          update_from_signet keyfile
         elsif keyfile.is_a? Hash
-          hash = stringify_hash_keys keyfile
-          hash["scope"] ||= scope
-          @client = init_client hash, options
-          @project_id ||= (hash["project_id"] || hash["project"])
+          update_from_hash keyfile, options
         else
-          verify_keyfile_exists! keyfile
-          json = JSON.parse ::File.read(keyfile)
-          json["scope"] ||= scope
-          @project_id ||= (json["project_id"] || json["project"])
-          @client = init_client json, options
+          update_from_filepath keyfile, options
         end
         CredentialsLoader.warn_if_cloud_sdk_credentials @client.client_id
         @project_id ||= CredentialsLoader.load_gcloud_project_id
@@ -261,7 +258,6 @@ module Google
         @paths = nil
         @scope = nil
       end
-      # rubocop:enable Metrics/AbcSize
 
       ##
       # Creates a new Credentials instance with auth credentials acquired by searching the
@@ -370,6 +366,29 @@ module Google
           issuer:               options["client_email"],
           signing_key:          OpenSSL::PKey::RSA.new(options["private_key"]) }
       end
+
+      def update_from_signet client
+        @project_id ||= client.project_id if client.respond_to? :project_id
+        @quota_project_id ||= client.quota_project_id if client.respond_to? :quota_project_id
+        @client = client
+      end
+
+      def update_from_hash hash, options
+        hash = stringify_hash_keys hash
+        hash["scope"] ||= options[:scope]
+        @project_id ||= (hash["project_id"] || hash["project"])
+        @quota_project_id ||= hash["quota_project_id"]
+        @client = init_client hash, options
+      end
+
+      def update_from_filepath path, options
+        verify_keyfile_exists! path
+        json = JSON.parse ::File.read(path)
+        json["scope"] ||= options[:scope]
+        @project_id ||= (json["project_id"] || json["project"])
+        @quota_project_id ||= json["quota_project_id"]
+        @client = init_client json, options
+      end
     end
   end
 end
diff --git a/lib/googleauth/json_key_reader.rb b/lib/googleauth/json_key_reader.rb
index 4f5002f..471ee42 100644
--- a/lib/googleauth/json_key_reader.rb
+++ b/lib/googleauth/json_key_reader.rb
@@ -38,8 +38,12 @@ module Google
         json_key = MultiJson.load json_key_io.read
         raise "missing client_email" unless json_key.key? "client_email"
         raise "missing private_key" unless json_key.key? "private_key"
-        project_id = json_key["project_id"]
-        [json_key["private_key"], json_key["client_email"], project_id]
+        [
+          json_key["private_key"],
+          json_key["client_email"],
+          json_key["project_id"],
+          json_key["quota_project_id"]
+        ]
       end
     end
   end
diff --git a/lib/googleauth/service_account.rb b/lib/googleauth/service_account.rb
index be931c8..3eb4e81 100644
--- a/lib/googleauth/service_account.rb
+++ b/lib/googleauth/service_account.rb
@@ -51,6 +51,7 @@ module Google
       extend CredentialsLoader
       extend JsonKeyReader
       attr_reader :project_id
+      attr_reader :quota_project_id
 
       # Creates a ServiceAccountCredentials.
       #
@@ -59,11 +60,12 @@ module Google
       def self.make_creds options = {}
         json_key_io, scope = options.values_at :json_key_io, :scope
         if json_key_io
-          private_key, client_email, project_id = read_json_key json_key_io
+          private_key, client_email, project_id, quota_project_id = read_json_key json_key_io
         else
           private_key = unescape ENV[CredentialsLoader::PRIVATE_KEY_VAR]
           client_email = ENV[CredentialsLoader::CLIENT_EMAIL_VAR]
           project_id = ENV[CredentialsLoader::PROJECT_ID_VAR]
+          quota_project_id = nil
         end
         project_id ||= CredentialsLoader.load_gcloud_project_id
 
@@ -72,7 +74,8 @@ module Google
             scope:                scope,
             issuer:               client_email,
             signing_key:          OpenSSL::PKey::RSA.new(private_key),
-            project_id:           project_id)
+            project_id:           project_id,
+            quota_project_id:     quota_project_id)
           .configure_connection(options)
       end
 
@@ -87,6 +90,7 @@ module Google
 
       def initialize options = {}
         @project_id = options[:project_id]
+        @quota_project_id = options[:quota_project_id]
         super options
       end
 
@@ -133,6 +137,7 @@ module Google
       extend CredentialsLoader
       extend JsonKeyReader
       attr_reader :project_id
+      attr_reader :quota_project_id
 
       # make_creds proxies the construction of a credentials instance
       #
@@ -151,12 +156,13 @@ module Google
       def initialize options = {}
         json_key_io = options[:json_key_io]
         if json_key_io
-          @private_key, @issuer, @project_id =
+          @private_key, @issuer, @project_id, @quota_project_id =
             self.class.read_json_key json_key_io
         else
           @private_key = ENV[CredentialsLoader::PRIVATE_KEY_VAR]
           @issuer = ENV[CredentialsLoader::CLIENT_EMAIL_VAR]
           @project_id = ENV[CredentialsLoader::PROJECT_ID_VAR]
+          @quota_project_id = nil
         end
         @project_id ||= CredentialsLoader.load_gcloud_project_id
         @signing_key = OpenSSL::PKey::RSA.new @private_key
diff --git a/spec/googleauth/credentials_spec.rb b/spec/googleauth/credentials_spec.rb
index 14a6dd3..de29854 100644
--- a/spec/googleauth/credentials_spec.rb
+++ b/spec/googleauth/credentials_spec.rb
@@ -36,12 +36,13 @@ require "googleauth"
 describe Google::Auth::Credentials, :private do
   let :default_keyfile_hash do
     {
-      "private_key_id" => "testabc1234567890xyz",
-      "private_key"    => "-----BEGIN RSA PRIVATE KEY-----\nMIIBOwIBAAJBAOyi0Hy1l4Ym2m2o71Q0TF4O9E81isZEsX0bb+Bqz1SXEaSxLiXM\nUZE8wu0eEXivXuZg6QVCW/5l+f2+9UPrdNUCAwEAAQJAJkqubA/Chj3RSL92guy3\nktzeodarLyw8gF8pOmpuRGSiEo/OLTeRUMKKD1/kX4f9sxf3qDhB4e7dulXR1co/\nIQIhAPx8kMW4XTTL6lJYd2K5GrH8uBMp8qL5ya3/XHrBgw3dAiEA7+3Iw3ULTn2I\n1J34WlJ2D5fbzMzB4FAHUNEV7Ys3f1kCIQDtUahCMChrl7+H5t9QS+xrn77lRGhs\nB50pjvy95WXpgQIhAI2joW6JzTfz8fAapb+kiJ/h9Vcs1ZN3iyoRlNFb61JZAiA8\nNy5NyNrMVwtB/lfJf1dAK/p/Bwd8LZLtgM6PapRfgw==\n-----END RSA PRIVATE KEY-----\n",
-      "client_email"   => "credz-testabc1234567890xyz@developer.gserviceaccount.com",
-      "client_id"      => "credz-testabc1234567890xyz.apps.googleusercontent.com",
-      "type"           => "service_account",
-      "project_id"     => "a_project_id"
+      "private_key_id"   => "testabc1234567890xyz",
+      "private_key"      => "-----BEGIN RSA PRIVATE KEY-----\nMIIBOwIBAAJBAOyi0Hy1l4Ym2m2o71Q0TF4O9E81isZEsX0bb+Bqz1SXEaSxLiXM\nUZE8wu0eEXivXuZg6QVCW/5l+f2+9UPrdNUCAwEAAQJAJkqubA/Chj3RSL92guy3\nktzeodarLyw8gF8pOmpuRGSiEo/OLTeRUMKKD1/kX4f9sxf3qDhB4e7dulXR1co/\nIQIhAPx8kMW4XTTL6lJYd2K5GrH8uBMp8qL5ya3/XHrBgw3dAiEA7+3Iw3ULTn2I\n1J34WlJ2D5fbzMzB4FAHUNEV7Ys3f1kCIQDtUahCMChrl7+H5t9QS+xrn77lRGhs\nB50pjvy95WXpgQIhAI2joW6JzTfz8fAapb+kiJ/h9Vcs1ZN3iyoRlNFb61JZAiA8\nNy5NyNrMVwtB/lfJf1dAK/p/Bwd8LZLtgM6PapRfgw==\n-----END RSA PRIVATE KEY-----\n",
+      "client_email"     => "credz-testabc1234567890xyz@developer.gserviceaccount.com",
+      "client_id"        => "credz-testabc1234567890xyz.apps.googleusercontent.com",
+      "type"             => "service_account",
+      "project_id"       => "a_project_id",
+      "quota_project_id" => "b_project_id"
     }
   end
 
@@ -118,6 +119,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials1)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
 
     it "subclasses can use PATH_ENV_VARS to get keyfile path" do
@@ -153,6 +155,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials2)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
 
     it "subclasses can use JSON_ENV_VARS to get keyfile contents" do
@@ -190,6 +193,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials3)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
 
     it "subclasses can use DEFAULT_PATHS to get keyfile path" do
@@ -225,6 +229,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials4)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
 
     it "subclasses that find no matches default to Google::Auth.get_application_default" do
@@ -266,6 +271,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials5)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
   end
 
@@ -305,6 +311,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials11)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
 
     it "subclasses can use PATH_ENV_VARS to get keyfile path" do
@@ -339,6 +346,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials12)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
 
     it "subclasses can use JSON_ENV_VARS to get keyfile contents" do
@@ -375,6 +383,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials13)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
 
     it "subclasses can use DEFAULT_PATHS to get keyfile path" do
@@ -409,6 +418,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials14)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
 
     it "subclasses that find no matches default to Google::Auth.get_application_default" do
@@ -449,6 +459,7 @@ describe Google::Auth::Credentials, :private do
       expect(creds).to be_a_kind_of(TestCredentials15)
       expect(creds.client).to eq(mocked_signet)
       expect(creds.project_id).to eq(default_keyfile_hash["project_id"])
+      expect(creds.quota_project_id).to eq(default_keyfile_hash["quota_project_id"])
     end
   end
 
diff --git a/spec/googleauth/service_account_spec.rb b/spec/googleauth/service_account_spec.rb
index 3e138dc..9d7444d 100644
--- a/spec/googleauth/service_account_spec.rb
+++ b/spec/googleauth/service_account_spec.rb
@@ -112,12 +112,13 @@ describe Google::Auth::ServiceAccountCredentials do
   let(:client_email) { "app@developer.gserviceaccount.com" }
   let :cred_json do
     {
-      private_key_id: "a_private_key_id",
-      private_key:    @key.to_pem,
-      client_email:   client_email,
-      client_id:      "app.apps.googleusercontent.com",
-      type:           "service_account",
-      project_id:     "a_project_id"
+      private_key_id:   "a_private_key_id",
+      private_key:      @key.to_pem,
+      client_email:     client_email,
+      client_id:        "app.apps.googleusercontent.com",
+      type:             "service_account",
+      project_id:       "a_project_id",
+      quota_project_id: "b_project_id"
     }
   end
 
@@ -285,6 +286,7 @@ describe Google::Auth::ServiceAccountCredentials do
         ENV["APPDATA"] = dir
         credentials = @clz.from_well_known_path @scope
         expect(credentials.project_id).to eq(cred_json[:project_id])
+        expect(credentials.quota_project_id).to eq(cred_json[:quota_project_id])
       end
     end
 
@@ -476,6 +478,7 @@ describe Google::Auth::ServiceAccountJwtHeaderCredentials do
         ENV["APPDATA"] = dir
         credentials = clz.from_well_known_path @scope
         expect(credentials.project_id).to eq(cred_json[:project_id])
+        expect(credentials.quota_project_id).to be_nil
       end
     end
   end