diff --git a/crates/handlers/src/compat/logout_all.rs b/crates/handlers/src/compat/logout_all.rs
new file mode 100644
index 000000000..489521313
--- /dev/null
+++ b/crates/handlers/src/compat/logout_all.rs
@@ -0,0 +1,201 @@
+// Copyright 2025 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only
+// Please see LICENSE in the repository root for full details.
+
+use std::sync::LazyLock;
+
+use axum::{Json, response::IntoResponse};
+use axum_extra::typed_header::TypedHeader;
+use headers::{Authorization, authorization::Bearer};
+use hyper::StatusCode;
+use mas_axum_utils::record_error;
+use mas_data_model::TokenType;
+use mas_storage::{
+    BoxClock, BoxRepository, BoxRng, Clock, RepositoryAccess,
+    compat::{CompatAccessTokenRepository, CompatSessionFilter, CompatSessionRepository},
+    queue::{QueueJobRepositoryExt as _, SyncDevicesJob},
+};
+use opentelemetry::{Key, KeyValue, metrics::Counter};
+use serde::Deserialize;
+use thiserror::Error;
+use tracing::info;
+use ulid::Ulid;
+
+use super::{MatrixError, MatrixJsonBody};
+use crate::{BoundActivityTracker, METER, impl_from_error_for_route};
+
+static LOGOUT_ALL_COUNTER: LazyLock<Counter<u64>> = LazyLock::new(|| {
+    METER
+        .u64_counter("mas.compat.logout_all_request")
+        .with_description(
+            "How many request to the /logout/all compatibility endpoint have happened",
+        )
+        .with_unit("{request}")
+        .build()
+});
+const RESULT: Key = Key::from_static_str("result");
+
+#[derive(Error, Debug)]
+pub enum RouteError {
+    #[error(transparent)]
+    Internal(Box<dyn std::error::Error + Send + Sync + 'static>),
+
+    #[error("Can't load session {0}")]
+    CantLoadSession(Ulid),
+
+    #[error("Can't load user {0}")]
+    CantLoadUser(Ulid),
+
+    #[error("Token {0} has expired")]
+    InvalidToken(Ulid),
+
+    #[error("Session {0} has been revoked")]
+    InvalidSession(Ulid),
+
+    #[error("User {0} is locked or deactivated")]
+    InvalidUser(Ulid),
+
+    #[error("/logout/all is not supported")]
+    NotSupported,
+
+    #[error("Missing access token")]
+    MissingAuthorization,
+
+    #[error("Invalid token format")]
+    TokenFormat(#[from] mas_data_model::TokenFormatError),
+
+    #[error("Access token is not a compatibility access token")]
+    NotACompatToken,
+}
+
+impl_from_error_for_route!(mas_storage::RepositoryError);
+
+impl IntoResponse for RouteError {
+    fn into_response(self) -> axum::response::Response {
+        let sentry_event_id = record_error!(
+            self,
+            Self::Internal(_) | Self::CantLoadSession(_) | Self::CantLoadUser(_)
+        );
+
+        // We track separately if the endpoint was called without the custom
+        // parameter, so that we know if clients are using this endpoint in the
+        // wild
+        if matches!(self, Self::NotSupported) {
+            LOGOUT_ALL_COUNTER.add(1, &[KeyValue::new(RESULT, "not_supported")]);
+        } else {
+            LOGOUT_ALL_COUNTER.add(1, &[KeyValue::new(RESULT, "error")]);
+        }
+
+        let response = match self {
+            Self::Internal(_) | Self::CantLoadSession(_) | Self::CantLoadUser(_) => MatrixError {
+                errcode: "M_UNKNOWN",
+                error: "Internal error",
+                status: StatusCode::INTERNAL_SERVER_ERROR,
+            },
+            Self::MissingAuthorization => MatrixError {
+                errcode: "M_MISSING_TOKEN",
+                error: "Missing access token",
+                status: StatusCode::UNAUTHORIZED,
+            },
+            Self::InvalidUser(_)
+            | Self::InvalidSession(_)
+            | Self::InvalidToken(_)
+            | Self::NotACompatToken
+            | Self::TokenFormat(_) => MatrixError {
+                errcode: "M_UNKNOWN_TOKEN",
+                error: "Invalid access token",
+                status: StatusCode::UNAUTHORIZED,
+            },
+            Self::NotSupported => MatrixError {
+                errcode: "M_UNRECOGNIZED",
+                error: "The /logout/all endpoint is not supported by this deployment",
+                status: StatusCode::NOT_FOUND,
+            },
+        };
+
+        (sentry_event_id, response).into_response()
+    }
+}
+
+#[derive(Deserialize, Default)]
+pub(crate) struct RequestBody {
+    #[serde(rename = "io.element.only_compat_is_fine", default)]
+    only_compat_is_fine: bool,
+}
+
+#[tracing::instrument(name = "handlers.compat.logout_all.post", skip_all)]
+pub(crate) async fn post(
+    clock: BoxClock,
+    mut rng: BoxRng,
+    mut repo: BoxRepository,
+    activity_tracker: BoundActivityTracker,
+    maybe_authorization: Option<TypedHeader<Authorization<Bearer>>>,
+    input: Option<MatrixJsonBody<RequestBody>>,
+) -> Result<impl IntoResponse, RouteError> {
+    let MatrixJsonBody(input) = input.unwrap_or_default();
+    let TypedHeader(authorization) = maybe_authorization.ok_or(RouteError::MissingAuthorization)?;
+
+    let token = authorization.token();
+    let token_type = TokenType::check(token)?;
+
+    if token_type != TokenType::CompatAccessToken {
+        return Err(RouteError::NotACompatToken);
+    }
+
+    let token = repo
+        .compat_access_token()
+        .find_by_token(token)
+        .await?
+        .ok_or(RouteError::NotACompatToken)?;
+
+    if !token.is_valid(clock.now()) {
+        return Err(RouteError::InvalidToken(token.id));
+    }
+
+    let session = repo
+        .compat_session()
+        .lookup(token.session_id)
+        .await?
+        .ok_or(RouteError::CantLoadSession(token.session_id))?;
+
+    if !session.is_valid() {
+        return Err(RouteError::InvalidSession(session.id));
+    }
+
+    activity_tracker
+        .record_compat_session(&clock, &session)
+        .await;
+
+    let user = repo
+        .user()
+        .lookup(session.user_id)
+        .await?
+        .ok_or(RouteError::CantLoadUser(session.user_id))?;
+
+    if !user.is_valid() {
+        return Err(RouteError::InvalidUser(session.user_id));
+    }
+
+    if !input.only_compat_is_fine {
+        return Err(RouteError::NotSupported);
+    }
+
+    let filter = CompatSessionFilter::new().for_user(&user).active_only();
+    let affected_sessions = repo.compat_session().finish_bulk(&clock, filter).await?;
+    info!(
+        "Logged out {affected_sessions} sessions for user {user_id}",
+        user_id = user.id
+    );
+
+    // Schedule a job to sync the devices of the user with the homeserver
+    repo.queue_job()
+        .schedule_job(&mut rng, &clock, SyncDevicesJob::new(&user))
+        .await?;
+
+    repo.save().await?;
+
+    LOGOUT_ALL_COUNTER.add(1, &[KeyValue::new(RESULT, "success")]);
+
+    Ok(Json(serde_json::json!({})))
+}
diff --git a/crates/handlers/src/compat/mod.rs b/crates/handlers/src/compat/mod.rs
index 304499471..abf02a28c 100644
--- a/crates/handlers/src/compat/mod.rs
+++ b/crates/handlers/src/compat/mod.rs
@@ -1,4 +1,4 @@
-// Copyright 2024 New Vector Ltd.
+// Copyright 2024, 2025 New Vector Ltd.
 // Copyright 2022-2024 The Matrix.org Foundation C.I.C.
 //
 // SPDX-License-Identifier: AGPL-3.0-only
@@ -22,6 +22,7 @@ pub(crate) mod login;
 pub(crate) mod login_sso_complete;
 pub(crate) mod login_sso_redirect;
 pub(crate) mod logout;
+pub(crate) mod logout_all;
 pub(crate) mod refresh;
 
 #[derive(Debug, Serialize)]
@@ -140,3 +141,29 @@ where
         Ok(Self(value))
     }
 }
+
+impl<T, S> axum::extract::OptionalFromRequest<S> for MatrixJsonBody<T>
+where
+    T: DeserializeOwned,
+    S: Send + Sync,
+{
+    type Rejection = MatrixJsonBodyRejection;
+
+    async fn from_request(req: Request, state: &S) -> Result<Option<Self>, Self::Rejection> {
+        if req.headers().contains_key(header::CONTENT_TYPE) {
+            // If there is a Content-Type header, handle it as normal
+            let result = <Self as axum::extract::FromRequest<S>>::from_request(req, state).await?;
+            return Ok(Some(result));
+        }
+
+        // Else, we poke at the body, and deserialize it only if it's JSON
+        let bytes = <Bytes as axum::extract::FromRequest<S>>::from_request(req, state).await?;
+        if bytes.is_empty() {
+            return Ok(None);
+        }
+
+        let value: T = serde_json::from_slice(&bytes)?;
+
+        Ok(Some(Self(value)))
+    }
+}
diff --git a/crates/handlers/src/lib.rs b/crates/handlers/src/lib.rs
index 5b8a5e48a..ed96a1a83 100644
--- a/crates/handlers/src/lib.rs
+++ b/crates/handlers/src/lib.rs
@@ -278,6 +278,10 @@ where
             mas_router::CompatLogout::route(),
             post(self::compat::logout::post),
         )
+        .route(
+            mas_router::CompatLogoutAll::route(),
+            post(self::compat::logout_all::post),
+        )
         .route(
             mas_router::CompatRefresh::route(),
             post(self::compat::refresh::post),
diff --git a/crates/router/src/endpoints.rs b/crates/router/src/endpoints.rs
index 89a9d726e..a7efeade9 100644
--- a/crates/router/src/endpoints.rs
+++ b/crates/router/src/endpoints.rs
@@ -548,6 +548,13 @@ impl SimpleRoute for CompatLogout {
     const PATH: &'static str = "/_matrix/client/{version}/logout";
 }
 
+/// `POST /_matrix/client/v3/logout/all`
+pub struct CompatLogoutAll;
+
+impl SimpleRoute for CompatLogoutAll {
+    const PATH: &'static str = "/_matrix/client/{version}/logout/all";
+}
+
 /// `POST /_matrix/client/v3/refresh`
 pub struct CompatRefresh;