diff --git a/internal/creds.go b/internal/creds.go
index 69b186b70a7..63c66092203 100644
--- a/internal/creds.go
+++ b/internal/creds.go
@@ -13,6 +13,7 @@ import (
 	"io/ioutil"
 	"net"
 	"net/http"
+	"os"
 	"time"
 
 	"golang.org/x/oauth2"
@@ -21,6 +22,8 @@ import (
 	"golang.org/x/oauth2/google"
 )
 
+const quotaProjectEnvVar = "GOOGLE_CLOUD_QUOTA_PROJECT"
+
 // Creds returns credential information obtained from DialSettings, or if none, then
 // it returns default credential information.
 func Creds(ctx context.Context, ds *DialSettings) (*google.Credentials, error) {
@@ -152,14 +155,22 @@ func selfSignedJWTTokenSource(data []byte, ds *DialSettings) (oauth2.TokenSource
 	}
 }
 
-// QuotaProjectFromCreds returns the quota project from the JSON blob in the provided credentials.
-//
-// NOTE(cbro): consider promoting this to a field on google.Credentials.
-func QuotaProjectFromCreds(cred *google.Credentials) string {
+// GetQuotaProject retrieves quota project with precedence being: client option,
+// environment variable, creds file.
+func GetQuotaProject(creds *google.Credentials, clientOpt string) string {
+	if clientOpt != "" {
+		return clientOpt
+	}
+	if env := os.Getenv(quotaProjectEnvVar); env != "" {
+		return env
+	}
+	if creds == nil {
+		return ""
+	}
 	var v struct {
 		QuotaProject string `json:"quota_project_id"`
 	}
-	if err := json.Unmarshal(cred.JSON, &v); err != nil {
+	if err := json.Unmarshal(creds.JSON, &v); err != nil {
 		return ""
 	}
 	return v.QuotaProject
diff --git a/internal/creds_test.go b/internal/creds_test.go
index 34b052dcb29..7b5bf2235d3 100644
--- a/internal/creds_test.go
+++ b/internal/creds_test.go
@@ -6,6 +6,7 @@ package internal
 
 import (
 	"context"
+	"os"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -199,10 +200,9 @@ const validServiceAccountJSON = `{
   "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/dumba-504%40appspot.gserviceaccount.com"
 }`
 
-func TestQuotaProjectFromCreds(t *testing.T) {
+func TestGetQuotaProject(t *testing.T) {
 	ctx := context.Background()
-
-	cred, err := credentialsFromJSON(
+	emptyCred, err := credentialsFromJSON(
 		ctx,
 		[]byte(validServiceAccountJSON),
 		&DialSettings{
@@ -212,17 +212,13 @@ func TestQuotaProjectFromCreds(t *testing.T) {
 	if err != nil {
 		t.Fatalf("got %v, wanted no error", err)
 	}
-	if want, got := "", QuotaProjectFromCreds(cred); want != got {
-		t.Errorf("QuotaProjectFromCreds(validServiceAccountJSON): want %q, got %q", want, got)
-	}
-
 	quotaProjectJSON := []byte(`
 {
 	"type": "authorized_user",
 	"quota_project_id": "foobar"
 }`)
 
-	cred, err = credentialsFromJSON(
+	quotaCred, err := credentialsFromJSON(
 		ctx,
 		[]byte(quotaProjectJSON),
 		&DialSettings{
@@ -232,8 +228,53 @@ func TestQuotaProjectFromCreds(t *testing.T) {
 	if err != nil {
 		t.Fatalf("got %v, wanted no error", err)
 	}
-	if want, got := "foobar", QuotaProjectFromCreds(cred); want != got {
-		t.Errorf("QuotaProjectFromCreds(quotaProjectJSON): want %q, got %q", want, got)
+
+	tests := []struct {
+		name      string
+		cred      *google.Credentials
+		clientOpt string
+		env       string
+		want      string
+	}{
+		{
+			name: "empty all",
+			cred: nil,
+			want: "",
+		},
+		{
+			name: "empty cred",
+			cred: emptyCred,
+			want: "",
+		},
+		{
+			name: "from cred",
+			cred: quotaCred,
+			want: "foobar",
+		},
+		{
+			name:      "from opt",
+			cred:      quotaCred,
+			clientOpt: "clientopt",
+			want:      "clientopt",
+		},
+		{
+			name: "from env",
+			cred: quotaCred,
+			env:  "envProject",
+			want: "envProject",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			oldEnv := os.Getenv(quotaProjectEnvVar)
+			if tt.env != "" {
+				os.Setenv(quotaProjectEnvVar, tt.env)
+			}
+			if want, got := tt.want, GetQuotaProject(tt.cred, tt.clientOpt); want != got {
+				t.Errorf("GetQuotaProject(%v, %q): want %q, got %q", tt.cred, tt.clientOpt, want, got)
+			}
+			os.Setenv(quotaProjectEnvVar, oldEnv)
+		})
 	}
 }
 
diff --git a/transport/grpc/dial.go b/transport/grpc/dial.go
index c76894ff4c6..20c94fa640b 100644
--- a/transport/grpc/dial.go
+++ b/transport/grpc/dial.go
@@ -154,14 +154,10 @@ func dial(ctx context.Context, insecure bool, o *internal.DialSettings) (*grpc.C
 			return nil, err
 		}
 
-		if o.QuotaProject == "" {
-			o.QuotaProject = internal.QuotaProjectFromCreds(creds)
-		}
-
 		grpcOpts = append(grpcOpts,
 			grpc.WithPerRPCCredentials(grpcTokenSource{
 				TokenSource:   oauth.TokenSource{creds.TokenSource},
-				quotaProject:  o.QuotaProject,
+				quotaProject:  internal.GetQuotaProject(creds, o.QuotaProject),
 				requestReason: o.RequestReason,
 			}),
 		)
diff --git a/transport/http/dial.go b/transport/http/dial.go
index 4f7f44e8dbf..403509d08f6 100644
--- a/transport/http/dial.go
+++ b/transport/http/dial.go
@@ -65,7 +65,6 @@ func newTransport(ctx context.Context, base http.RoundTripper, settings *interna
 	paramTransport := &parameterTransport{
 		base:          base,
 		userAgent:     settings.UserAgent,
-		quotaProject:  settings.QuotaProject,
 		requestReason: settings.RequestReason,
 	}
 	var trans http.RoundTripper = paramTransport
@@ -74,6 +73,7 @@ func newTransport(ctx context.Context, base http.RoundTripper, settings *interna
 	case settings.NoAuth:
 		// Do nothing.
 	case settings.APIKey != "":
+		paramTransport.quotaProject = internal.GetQuotaProject(nil, settings.QuotaProject)
 		trans = &transport.APIKey{
 			Transport: trans,
 			Key:       settings.APIKey,
@@ -83,10 +83,7 @@ func newTransport(ctx context.Context, base http.RoundTripper, settings *interna
 		if err != nil {
 			return nil, err
 		}
-		if paramTransport.quotaProject == "" {
-			paramTransport.quotaProject = internal.QuotaProjectFromCreds(creds)
-		}
-
+		paramTransport.quotaProject = internal.GetQuotaProject(creds, settings.QuotaProject)
 		ts := creds.TokenSource
 		if settings.ImpersonationConfig == nil && settings.TokenSource != nil {
 			ts = settings.TokenSource