diff --git a/include/mtx/user_interactive.hpp b/include/mtx/user_interactive.hpp
index 008734f39..e334aed8e 100644
--- a/include/mtx/user_interactive.hpp
+++ b/include/mtx/user_interactive.hpp
@@ -40,6 +40,8 @@ constexpr std::string_view sso = "m.login.sso"; // needed for /login at least
 constexpr std::string_view dummy = "m.login.dummy";
 //! Authentication by accepting a set of terms like a privacy policy.
 constexpr std::string_view terms = "m.login.terms"; // see MSC1692
+//! Authentication using a registration token. See MSC3231.
+constexpr std::string_view registration_token = "org.matrix.msc3231.login.registration_token";
 }
 
 //! A list of auth types
@@ -181,6 +183,13 @@ struct MSISDN
         std::vector<ThreePIDCred> threepidCreds;
 };
 
+//! Registration token authentication stage.
+struct RegistrationToken
+{
+        //! The registration token to use
+        std::string token;
+};
+
 //! OAuth2, client retries with the session only, so I'm guessing this is empty?
 struct OAuth2
 {};
@@ -214,6 +223,7 @@ struct Auth
                      auth::Terms,
                      auth::SSO,
                      auth::Dummy,
+                     auth::RegistrationToken,
                      auth::Fallback>
           content;
 };
diff --git a/lib/structs/user_interactive.cpp b/lib/structs/user_interactive.cpp
index af6861065..e3b18fea6 100644
--- a/lib/structs/user_interactive.cpp
+++ b/lib/structs/user_interactive.cpp
@@ -117,6 +117,10 @@ to_json(nlohmann::json &obj, const Auth &auth)
                              obj["type"]          = auth_types::msisdn;
                              obj["threepidCreds"] = id.threepidCreds;
                      },
+                     [&obj](const auth::RegistrationToken &registration_token) {
+                             obj["type"]  = auth_types::registration_token;
+                             obj["token"] = registration_token.token;
+                     },
                      [&obj](const auth::OAuth2 &) { obj["type"] = auth_types::oauth2; },
                      [&obj](const auth::SSO &) { obj["type"] = auth_types::sso; },
                      [&obj](const auth::Terms &) { obj["type"] = auth_types::terms; },
diff --git a/tests/requests.cpp b/tests/requests.cpp
index fa24be3ac..1e491b023 100644
--- a/tests/requests.cpp
+++ b/tests/requests.cpp
@@ -281,6 +281,13 @@ TEST(Requests, UserInteractiveAuth)
   ],
   "session": "<session ID>"
 })"_json);
+
+        a.content = auth::RegistrationToken{"<token>"};
+        EXPECT_EQ(nlohmann::json(a), R"({
+  "type": "org.matrix.msc3231.login.registration_token",
+  "token": "<token>",
+  "session": "<session ID>"
+})"_json);
 }
 
 TEST(Requests, RoomVisibility)