diff --git a/blob/gcsblob/gcsblob.go b/blob/gcsblob/gcsblob.go
index 322b7f396f..20f7a24bb0 100644
--- a/blob/gcsblob/gcsblob.go
+++ b/blob/gcsblob/gcsblob.go
@@ -14,6 +14,17 @@
 
 // Package gcsblob provides an implementation of blob that uses GCS.
 //
+// For blob.Open URLs, gcsblob registers for the "gs" protocol.
+// The URL's Host is used as the bucket name.
+// The following query options are supported:
+// - cred_path: Sets path to the Google credentials file. If unset, default
+//       credentials are loaded.
+//       See https://cloud.google.com/docs/authentication/production.
+// - access_id: Sets Options.GoogleAccessID.
+// - private_key_path: Sets path to a private key, which is read and used
+//       to set Options.PrivateKey.
+// Example URL: blob.Open("gs://mybucket")
+//
 // It exposes the following types for As:
 // Bucket: *storage.Client
 // ListObject: storage.ObjectAttrs
@@ -28,6 +39,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"net/url"
 	"sort"
 	"strings"
 	"time"
@@ -37,6 +49,7 @@ import (
 	"github.com/google/go-cloud/gcp"
 
 	"cloud.google.com/go/storage"
+	"golang.org/x/oauth2/google"
 	"google.golang.org/api/googleapi"
 	"google.golang.org/api/iterator"
 	"google.golang.org/api/option"
@@ -44,6 +57,49 @@ import (
 
 const defaultPageSize = 1000
 
+func init() {
+	blob.Register("gs", func(ctx context.Context, u *url.URL) (driver.Bucket, error) {
+		q := u.Query()
+		opts := &Options{}
+
+		if accessID := q["access_id"]; len(accessID) > 0 {
+			opts.GoogleAccessID = accessID[0]
+		}
+
+		if keyPath := q["private_key_path"]; len(keyPath) > 0 {
+			pk, err := ioutil.ReadFile(keyPath[0])
+			if err != nil {
+				return nil, fmt.Errorf("reading private key: %v", err)
+			}
+			opts.PrivateKey = pk
+		}
+
+		var creds *google.Credentials
+		if credPath := q["cred_path"]; len(credPath) == 0 {
+			var err error
+			creds, err = gcp.DefaultCredentials(ctx)
+			if err != nil {
+				return nil, err
+			}
+		} else {
+			jsonCreds, err := ioutil.ReadFile(credPath[0])
+			if err != nil {
+				return nil, fmt.Errorf("reading credentials: %v", err)
+			}
+			creds, err = google.CredentialsFromJSON(ctx, jsonCreds)
+			if err != nil {
+				return nil, fmt.Errorf("loading credentials: %v", err)
+			}
+		}
+
+		client, err := gcp.NewHTTPClient(gcp.DefaultTransport(), gcp.CredentialsTokenSource(creds))
+		if err != nil {
+			return nil, err
+		}
+		return openBucket(ctx, u.Host, client, opts)
+	})
+}
+
 // Options sets options for constructing a *blob.Bucket backed by GCS.
 type Options struct {
 	// GoogleAccessID represents the authorizer for SignedURL.
@@ -62,8 +118,8 @@ type Options struct {
 	SignBytes func([]byte) ([]byte, error)
 }
 
-// OpenBucket returns a GCS Bucket that communicates using the given HTTP client.
-func OpenBucket(ctx context.Context, bucketName string, client *gcp.HTTPClient, opts *Options) (*blob.Bucket, error) {
+// openBucket returns a GCS Bucket that communicates using the given HTTP client.
+func openBucket(ctx context.Context, bucketName string, client *gcp.HTTPClient, opts *Options) (driver.Bucket, error) {
 	if client == nil {
 		return nil, fmt.Errorf("OpenBucket requires an HTTP client")
 	}
@@ -74,7 +130,16 @@ func OpenBucket(ctx context.Context, bucketName string, client *gcp.HTTPClient,
 	if opts == nil {
 		opts = &Options{}
 	}
-	return blob.NewBucket(&bucket{name: bucketName, client: c, opts: opts}), nil
+	return &bucket{name: bucketName, client: c, opts: opts}, nil
+}
+
+// OpenBucket returns a GCS Bucket that communicates using the given HTTP client.
+func OpenBucket(ctx context.Context, bucketName string, client *gcp.HTTPClient, opts *Options) (*blob.Bucket, error) {
+	drv, err := openBucket(ctx, bucketName, client, opts)
+	if err != nil {
+		return nil, err
+	}
+	return blob.NewBucket(drv), nil
 }
 
 // bucket represents a GCS bucket, which handles read, write and delete operations
diff --git a/blob/s3blob/s3blob.go b/blob/s3blob/s3blob.go
index 38710601ab..b182b7c0e3 100644
--- a/blob/s3blob/s3blob.go
+++ b/blob/s3blob/s3blob.go
@@ -14,7 +14,15 @@
 
 // Package s3blob provides an implementation of blob using S3.
 //
-// It exposes the following types for As:
+// For blob.Open URLs, s3blob registers for the "s3" protocol.
+// The URL's Host is used as the bucket name.
+// The AWS session is created as described in
+// https://docs.aws.amazon.com/sdk-for-go/api/aws/session/.
+// The following query options are supported:
+// - region: The AWS region for requests; sets aws.Config.Region.
+// Example URL: blob.Open("s3://mybucket?region=us-east-1")
+//
+// s3blob exposes the following types for As:
 // Bucket: *s3.S3
 // ListObject: s3.Object for objects, s3.CommonPrefix for "directories".
 // ListOptions.BeforeList: *s3.ListObjectsV2Input
@@ -30,6 +38,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"net/url"
 	"sort"
 	"strconv"
 	"strings"
@@ -40,25 +49,51 @@ import (
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/awserr"
 	"github.com/aws/aws-sdk-go/aws/client"
+	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/s3"
 	"github.com/aws/aws-sdk-go/service/s3/s3manager"
 )
 
 const defaultPageSize = 1000
 
+func init() {
+	blob.Register("s3", func(ctx context.Context, u *url.URL) (driver.Bucket, error) {
+		q := u.Query()
+		cfg := &aws.Config{}
+
+		if region := q["region"]; len(region) > 0 {
+			cfg.Region = aws.String(region[0])
+		}
+		sess, err := session.NewSession(cfg)
+		if err != nil {
+			return nil, err
+		}
+		return openBucket(ctx, u.Host, sess, nil)
+	})
+}
+
 // Options sets options for constructing a *blob.Bucket backed by fileblob.
 type Options struct{}
 
-// OpenBucket returns an S3 Bucket.
-func OpenBucket(ctx context.Context, bucketName string, sess client.ConfigProvider, _ *Options) (*blob.Bucket, error) {
+// openBucket returns an S3 Bucket.
+func openBucket(ctx context.Context, bucketName string, sess client.ConfigProvider, _ *Options) (driver.Bucket, error) {
 	if sess == nil {
 		return nil, errors.New("sess must be provided to get bucket")
 	}
-	return blob.NewBucket(&bucket{
+	return &bucket{
 		name:   bucketName,
 		sess:   sess,
 		client: s3.New(sess),
-	}), nil
+	}, nil
+}
+
+// OpenBucket returns an S3 Bucket.
+func OpenBucket(ctx context.Context, bucketName string, sess client.ConfigProvider, opts *Options) (*blob.Bucket, error) {
+	drv, err := openBucket(ctx, bucketName, sess, opts)
+	if err != nil {
+		return nil, err
+	}
+	return blob.NewBucket(drv), nil
 }
 
 var emptyBody = ioutil.NopCloser(strings.NewReader(""))
diff --git a/blob/s3blob/s3blob_test.go b/blob/s3blob/s3blob_test.go
index f44bdb6cad..a12fb747cc 100644
--- a/blob/s3blob/s3blob_test.go
+++ b/blob/s3blob/s3blob_test.go
@@ -59,7 +59,7 @@ func (h *harness) HTTPClient() *http.Client {
 }
 
 func (h *harness) MakeDriver(ctx context.Context) (driver.Bucket, error) {
-	return &bucket{name: bucketName, sess: h.session, client: s3.New(h.session)}, nil
+	return openBucket(ctx, bucketName, h.session, nil)
 }
 
 func (h *harness) Close() {