diff --git a/client.go b/client.go
index 6b60f9751..902056f3d 100644
--- a/client.go
+++ b/client.go
@@ -1043,6 +1043,22 @@ func (cli *Client) SetDisplayName(ctx context.Context, displayName string) (err
 	return
 }
 
+// UnstableSetProfileField sets an arbitrary MSC4133 profile field. See https://github.com/matrix-org/matrix-spec-proposals/pull/4133
+func (cli *Client) UnstableSetProfileField(ctx context.Context, key string, value any) (err error) {
+	urlPath := cli.BuildClientURL("unstable", "uk.tcpip.msc4133", "profile", cli.UserID, key)
+	_, err = cli.MakeRequest(ctx, http.MethodPut, urlPath, map[string]any{
+		key: value,
+	}, nil)
+	return
+}
+
+// UnstableDeleteProfileField deletes an arbitrary MSC4133 profile field. See https://github.com/matrix-org/matrix-spec-proposals/pull/4133
+func (cli *Client) UnstableDeleteProfileField(ctx context.Context, key string) (err error) {
+	urlPath := cli.BuildClientURL("unstable", "uk.tcpip.msc4133", "profile", cli.UserID, key)
+	_, err = cli.MakeRequest(ctx, http.MethodDelete, urlPath, nil, nil)
+	return
+}
+
 // GetAvatarURL gets the avatar URL of the user with the specified MXID. See https://spec.matrix.org/v1.2/client-server-api/#get_matrixclientv3profileuseridavatar_url
 func (cli *Client) GetAvatarURL(ctx context.Context, mxid id.UserID) (url id.ContentURI, err error) {
 	urlPath := cli.BuildClientURL("v3", "profile", mxid, "avatar_url")
diff --git a/responses.go b/responses.go
index 7312f0996..a2e5c2b86 100644
--- a/responses.go
+++ b/responses.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"maps"
 	"reflect"
 	"strconv"
 	"strings"
@@ -155,8 +156,44 @@ type RespUserDisplayName struct {
 }
 
 type RespUserProfile struct {
-	DisplayName string        `json:"displayname"`
-	AvatarURL   id.ContentURI `json:"avatar_url"`
+	DisplayName string         `json:"displayname,omitempty"`
+	AvatarURL   id.ContentURI  `json:"avatar_url,omitempty"`
+	Extra       map[string]any `json:"-"`
+}
+
+type marshalableUserProfile RespUserProfile
+
+func (r *RespUserProfile) UnmarshalJSON(data []byte) error {
+	err := json.Unmarshal(data, &r.Extra)
+	if err != nil {
+		return err
+	}
+	r.DisplayName, _ = r.Extra["displayname"].(string)
+	avatarURL, _ := r.Extra["avatar_url"].(string)
+	if avatarURL != "" {
+		r.AvatarURL, _ = id.ParseContentURI(avatarURL)
+	}
+	delete(r.Extra, "displayname")
+	delete(r.Extra, "avatar_url")
+	return nil
+}
+
+func (r *RespUserProfile) MarshalJSON() ([]byte, error) {
+	if len(r.Extra) == 0 {
+		return json.Marshal((*marshalableUserProfile)(r))
+	}
+	marshalMap := maps.Clone(r.Extra)
+	if r.DisplayName != "" {
+		marshalMap["displayname"] = r.DisplayName
+	} else {
+		delete(marshalMap, "displayname")
+	}
+	if !r.AvatarURL.IsEmpty() {
+		marshalMap["avatar_url"] = r.AvatarURL.String()
+	} else {
+		delete(marshalMap, "avatar_url")
+	}
+	return json.Marshal(r.Extra)
 }
 
 type RespMutualRooms struct {