diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index 0bda1e488..ce6076a71 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -19,6 +19,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	rsAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"io"
 	"net/http"
 	"net/url"
@@ -536,6 +537,7 @@ func validateApplicationService(
 func Register(
 	req *http.Request,
 	userAPI userapi.ClientUserAPI,
+	clientRsApi rsAPI.ClientRoomserverAPI,
 	cfg *config.ClientAPI,
 ) util.JSONResponse {
 	defer req.Body.Close() // nolint: errcheck
@@ -632,7 +634,7 @@ func Register(
 		"session_id": r.Auth.Session,
 	}).Info("Processing registration request")
 
-	return handleRegistrationFlow(req, r, sessionID, cfg, userAPI, accessToken, accessTokenErr)
+	return handleRegistrationFlow(req, r, sessionID, cfg, userAPI, accessToken, accessTokenErr, clientRsApi)
 }
 
 func handleGuestRegistration(
@@ -707,6 +709,7 @@ func handleRegistrationFlow(
 	userAPI userapi.ClientUserAPI,
 	accessToken string,
 	accessTokenErr error,
+	clientRsApi rsAPI.ClientRoomserverAPI,
 ) util.JSONResponse {
 	// TODO: Enable registration config flag
 	// TODO: Guest account upgrading
@@ -775,8 +778,7 @@ func handleRegistrationFlow(
 	// Check if the user's registration flow has been completed successfully
 	// A response with current registration flow and remaining available methods
 	// will be returned if a flow has not been successfully completed yet
-	return checkAndCompleteFlow(sessions.getCompletedStages(sessionID),
-		req, r, sessionID, cfg, userAPI)
+	return checkAndCompleteFlow(sessions.getCompletedStages(sessionID), req, r, sessionID, cfg, userAPI, clientRsApi)
 }
 
 // handleApplicationServiceRegistration handles the registration of an
@@ -825,15 +827,25 @@ func handleApplicationServiceRegistration(
 // checkAndCompleteFlow checks if a given registration flow is completed given
 // a set of allowed flows. If so, registration is completed, otherwise a
 // response with
-func checkAndCompleteFlow(
-	flow []authtypes.LoginType,
+func checkAndCompleteFlow(flow []authtypes.LoginType,
 	req *http.Request,
 	r registerRequest,
 	sessionID string,
 	cfg *config.ClientAPI,
 	userAPI userapi.ClientUserAPI,
-) util.JSONResponse {
+	clientRsApi rsAPI.ClientRoomserverAPI) util.JSONResponse {
+
 	if checkFlowCompleted(flow, cfg.Derived.Registration.Flows) {
+		// POST register behavior: add user to room
+		for room := range cfg.AutoJoinRooms {
+			err := addUserToRoom(req.Context(), clientRsApi, cfg.AutoJoinRooms[room], r.Username,
+				userutil.MakeUserID(r.Username, cfg.Matrix.ServerName))
+			if err != nil {
+				log.Fatal(err)
+				return util.JSONResponse{Code: http.StatusInternalServerError,
+					JSON: errorResponse(req.Context(), err, "Cannot add user to room.")}
+			}
+		}
 		// This flow was completed, registration can continue
 		return completeRegistration(
 			req.Context(), userAPI, r.Username, r.Password, "", req.RemoteAddr, req.UserAgent(), sessionID,
@@ -850,6 +862,24 @@ func checkAndCompleteFlow(
 	}
 }
 
+func addUserToRoom(
+	ctx context.Context,
+	clientRsAPI rsAPI.ClientRoomserverAPI,
+	roomID string,
+	username string,
+	userID string,
+) error {
+	addGroupContent := make(map[string]interface{})
+	addGroupContent["displayname"] = username
+	joinReq := rsAPI.PerformJoinRequest{
+		RoomIDOrAlias: roomID,
+		UserID:        userID,
+		Content:       addGroupContent,
+	}
+	joinRes := rsAPI.PerformJoinResponse{}
+	return clientRsAPI.PerformJoin(ctx, &joinReq, &joinRes)
+}
+
 // completeRegistration runs some rudimentary checks against the submitted
 // input, then if successful creates an account and a newly associated device
 // We pass in each individual part of the request here instead of just passing a
diff --git a/clientapi/routing/routing.go b/clientapi/routing/routing.go
index 4ca8e59c5..1a1ed05d0 100644
--- a/clientapi/routing/routing.go
+++ b/clientapi/routing/routing.go
@@ -432,7 +432,7 @@ func Setup(
 		if r := rateLimits.Limit(req, nil); r != nil {
 			return *r
 		}
-		return Register(req, userAPI, cfg)
+		return Register(req, userAPI, rsAPI, cfg)
 	})).Methods(http.MethodPost, http.MethodOptions)
 
 	v3mux.Handle("/register/available", httputil.MakeExternalAPI("registerAvailable", func(req *http.Request) util.JSONResponse {
diff --git a/dendrite-sample.monolith.yaml b/dendrite-sample.monolith.yaml
index eadb74a2a..c0f6fe99f 100644
--- a/dendrite-sample.monolith.yaml
+++ b/dendrite-sample.monolith.yaml
@@ -181,6 +181,9 @@ client_api:
   recaptcha_bypass_secret: ""
   recaptcha_siteverify_api: ""
 
+#  auto_join_rooms:
+#    - "#main:matrix.org"
+
   # TURN server information that this homeserver should send to clients.
   turn:
     turn_user_lifetime: "5m"
diff --git a/setup/config/config_clientapi.go b/setup/config/config_clientapi.go
index 56f4b3f92..76e062172 100644
--- a/setup/config/config_clientapi.go
+++ b/setup/config/config_clientapi.go
@@ -29,6 +29,10 @@ type ClientAPI struct {
 	// is forbidden either way.
 	GuestsDisabled bool `yaml:"guests_disabled"`
 
+	// Users who register on this homeserver will automatically
+	// be joined to the rooms listed under this option.
+	AutoJoinRooms []string `yaml:"auto_join_rooms"`
+
 	// Boolean stating whether catpcha registration is enabled
 	// and required
 	RecaptchaEnabled bool `yaml:"enable_registration_captcha"`