diff --git a/src/github.com/matrix-org/dendrite/clientapi/auth/storage/accounts/storage.go b/src/github.com/matrix-org/dendrite/clientapi/auth/storage/accounts/storage.go
index 16474ec6b..5449df5c5 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/auth/storage/accounts/storage.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/auth/storage/accounts/storage.go
@@ -314,3 +314,13 @@ func (d *Database) GetThreePIDsForLocalpart(
 ) (threepids []authtypes.ThreePID, err error) {
 	return d.threepids.selectThreePIDsForLocalpart(ctx, localpart)
 }
+
+// CheckAccountAvailability checks if the username/localpart is already present in the database.
+// If the DB returns sql.ErrNoRows the Localpart isn't taken.
+func (d *Database) CheckAccountAvailability(ctx context.Context, localpart string) (bool, error) {
+	_, err := d.accounts.selectAccountByLocalpart(ctx, localpart)
+	if err == sql.ErrNoRows {
+		return true, nil
+	}
+	return false, err
+}
diff --git a/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go b/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go
index b32ddf147..00bdd15f4 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go
@@ -131,6 +131,10 @@ func Setup(
 		return writers.LegacyRegister(req, accountDB, deviceDB, &cfg)
 	})).Methods("POST", "OPTIONS")
 
+	r0mux.Handle("/register/available", common.MakeExternalAPI("registerAvailable", func(req *http.Request) util.JSONResponse {
+		return writers.RegisterAvailable(req, accountDB)
+	})).Methods("GET")
+
 	r0mux.Handle("/directory/room/{roomAlias}",
 		common.MakeAuthAPI("directory_room", deviceDB, func(req *http.Request, device *authtypes.Device) util.JSONResponse {
 			vars := mux.Vars(req)
diff --git a/src/github.com/matrix-org/dendrite/clientapi/writers/register.go b/src/github.com/matrix-org/dendrite/clientapi/writers/register.go
index 8519c9a1f..84227d155 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/writers/register.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/writers/register.go
@@ -91,24 +91,14 @@ type registerResponse struct {
 	DeviceID    string                       `json:"device_id"`
 }
 
-// Validate returns an error response if the username/password are invalid
-func validate(username, password string) *util.JSONResponse {
+// validateUserName returns an error response if the username is invalid
+func validateUserName(username string) *util.JSONResponse {
 	// https://github.com/matrix-org/synapse/blob/v0.20.0/synapse/rest/client/v2_alpha/register.py#L161
-	if len(password) > maxPasswordLength {
-		return &util.JSONResponse{
-			Code: 400,
-			JSON: jsonerror.BadJSON(fmt.Sprintf("'password' >%d characters", maxPasswordLength)),
-		}
-	} else if len(username) > maxUsernameLength {
+	if len(username) > maxUsernameLength {
 		return &util.JSONResponse{
 			Code: 400,
 			JSON: jsonerror.BadJSON(fmt.Sprintf("'username' >%d characters", maxUsernameLength)),
 		}
-	} else if len(password) > 0 && len(password) < minPasswordLength {
-		return &util.JSONResponse{
-			Code: 400,
-			JSON: jsonerror.WeakPassword(fmt.Sprintf("password too weak: min %d chars", minPasswordLength)),
-		}
 	} else if !validUsernameRegex.MatchString(username) {
 		return &util.JSONResponse{
 			Code: 400,
@@ -123,6 +113,23 @@ func validate(username, password string) *util.JSONResponse {
 	return nil
 }
 
+// validatePassword returns an error response if the password is invalid
+func validatePassword(password string) *util.JSONResponse {
+	// https://github.com/matrix-org/synapse/blob/v0.20.0/synapse/rest/client/v2_alpha/register.py#L161
+	if len(password) > maxPasswordLength {
+		return &util.JSONResponse{
+			Code: 400,
+			JSON: jsonerror.BadJSON(fmt.Sprintf("'password' >%d characters", maxPasswordLength)),
+		}
+	} else if len(password) > 0 && len(password) < minPasswordLength {
+		return &util.JSONResponse{
+			Code: 400,
+			JSON: jsonerror.WeakPassword(fmt.Sprintf("password too weak: min %d chars", minPasswordLength)),
+		}
+	}
+	return nil
+}
+
 // Register processes a /register request. http://matrix.org/speculator/spec/HEAD/client_server/unstable.html#post-matrix-client-unstable-register
 func Register(
 	req *http.Request,
@@ -149,7 +156,10 @@ func Register(
 		}
 	}
 
-	if resErr = validate(r.Username, r.Password); resErr != nil {
+	if resErr = validateUserName(r.Username); resErr != nil {
+		return *resErr
+	}
+	if resErr = validatePassword(r.Password); resErr != nil {
 		return *resErr
 	}
 
@@ -209,7 +219,10 @@ func LegacyRegister(
 	if resErr != nil {
 		return *resErr
 	}
-	if resErr = validate(r.Username, r.Password); resErr != nil {
+	if resErr = validateUserName(r.Username); resErr != nil {
+		return *resErr
+	}
+	if resErr = validatePassword(r.Password); resErr != nil {
 		return *resErr
 	}
 
@@ -344,3 +357,40 @@ func isValidMacLogin(
 
 	return hmac.Equal(givenMac, expectedMAC), nil
 }
+
+type availableResponse struct {
+	Available bool `json:"available"`
+}
+
+// RegisterAvailable checks if the username is already taken or invalid
+func RegisterAvailable(
+	req *http.Request,
+	accountDB *accounts.Database,
+) util.JSONResponse {
+	username := req.URL.Query().Get("username")
+
+	if err := validateUserName(username); err != nil {
+		return *err
+	}
+
+	availability, availabilityErr := accountDB.CheckAccountAvailability(req.Context(), username)
+	if availabilityErr != nil {
+		return util.JSONResponse{
+			Code: 500,
+			JSON: jsonerror.Unknown("failed to check availability: " + availabilityErr.Error()),
+		}
+	}
+	if !availability {
+		return util.JSONResponse{
+			Code: 400,
+			JSON: jsonerror.InvalidUsername("A different user ID has already been registered for this session"),
+		}
+	}
+
+	return util.JSONResponse{
+		Code: 200,
+		JSON: availableResponse{
+			Available: true,
+		},
+	}
+}