diff --git a/src/transaction/tests/mod.rs b/src/transaction/tests/mod.rs
index 54e977d0..ecf2ff93 100644
--- a/src/transaction/tests/mod.rs
+++ b/src/transaction/tests/mod.rs
@@ -10,6 +10,7 @@ mod test_client;
 mod test_endpoint;
 mod test_provisional_responses;
 mod test_server;
+mod test_server_invite_ack;
 mod test_server_invite_drop;
 mod test_transaction_states;
 
diff --git a/src/transaction/tests/test_server_invite_ack.rs b/src/transaction/tests/test_server_invite_ack.rs
new file mode 100644
index 00000000..8a1ea20e
--- /dev/null
+++ b/src/transaction/tests/test_server_invite_ack.rs
@@ -0,0 +1,250 @@
+//! Tests for ACK matching on a server INVITE transaction (RFC 3261 §17.1.1.3,
+//! §13.2.2.4): an ACK acknowledges only the INVITE with the same CSeq number.
+//!
+//! ACKs for 2xx are routed per dialog through `waiting_ack`, so a delayed ACK
+//! of an earlier (re-)INVITE on the same dialog reaches the transaction of the
+//! current re-INVITE. It must not confirm that transaction or stop its 2xx
+//! retransmissions (Timer G).
+
+use crate::sip::headers::*;
+use crate::sip::prelude::HeadersExt;
+use crate::sip::{Method, SipMessage, StatusCode, Version};
+use crate::transaction::TransactionState;
+use crate::transport::udp::UdpConnection;
+use crate::transport::{SipConnection, TransportLayer};
+use crate::EndpointBuilder;
+use std::time::Duration;
+use tokio::time::timeout;
+use tokio_util::sync::CancellationToken;
+
+fn make_reinvite(transport: &str, cseq: u32) -> crate::sip::Request {
+    crate::sip::Request {
+        method: Method::Invite,
+        uri: crate::sip::Uri::try_from("sip:bob@127.0.0.1").unwrap(),
+        headers: vec![
+            Via::new(format!(
+                "SIP/2.0/{} 127.0.0.1:5060;branch=z9hG4bK-reinvite-{}",
+                transport, cseq
+            ))
+            .into(),
+            CSeq::new(format!("{} INVITE", cseq)).into(),
+            From::new("Alice <sip:alice@example.com>;tag=aliceTagAck").into(),
+            To::new("Bob <sip:bob@example.com>;tag=bobTagAck").into(),
+            CallId::new("ack-cseq-test@example.com").into(),
+            MaxForwards::new("70").into(),
+            ContentLength::new("0").into(),
+        ]
+        .into(),
+        version: Version::V2,
+        body: Default::default(),
+    }
+}
+
+/// A 2xx ACK on the dialog of `invite`: new branch, CSeq number `cseq`.
+fn make_2xx_ack(invite: &crate::sip::Request, cseq: u32) -> crate::sip::Request {
+    let mut headers = invite.headers.clone();
+    for header in headers.iter_mut() {
+        match header {
+            crate::sip::Header::Via(via) => {
+                let sent_by = via.value().split(';').next().unwrap().to_string();
+                *via = Via::new(format!("{};branch=z9hG4bK-ack-{}", sent_by, cseq));
+            }
+            crate::sip::Header::CSeq(c) => {
+                *c = CSeq::new(format!("{} ACK", cseq));
+            }
+            _ => {}
+        }
+    }
+    crate::sip::Request {
+        method: Method::Ack,
+        uri: invite.uri.clone(),
+        headers,
+        version: invite.version,
+        body: Default::default(),
+    }
+}
+
+/// A delayed ACK of a previous re-INVITE (CSeq 1) must not confirm the
+/// transaction of the current re-INVITE (CSeq 2): the 2xx keeps being
+/// retransmitted (Timer G), and the ACK with the matching CSeq still
+/// confirms the transaction.
+#[tokio::test]
+async fn test_server_invite_ignores_ack_with_other_cseq() {
+    let token = CancellationToken::new();
+
+    let server_conn = UdpConnection::create_connection("127.0.0.1:0".parse().unwrap(), None, None)
+        .await
+        .expect("create server connection");
+    let server_conn_sip: SipConnection = server_conn.into();
+    let server_addr = server_conn_sip.get_addr().clone();
+
+    let tl = TransportLayer::new(token.child_token());
+    tl.add_transport(server_conn_sip);
+
+    let endpoint = EndpointBuilder::new()
+        .with_user_agent("rsipstack-test")
+        .with_transport_layer(tl)
+        .build();
+
+    let client_conn = UdpConnection::create_connection("127.0.0.1:0".parse().unwrap(), None, None)
+        .await
+        .expect("create client connection");
+    let client_conn_sip: SipConnection = client_conn.clone().into();
+
+    let endpoint_inner = endpoint.inner.clone();
+    let serve_handle = tokio::spawn(async move {
+        let _ = endpoint_inner.serve().await;
+    });
+
+    // Re-INVITE with CSeq 2 on an established dialog.
+    let invite = make_reinvite("UDP", 2);
+    client_conn_sip
+        .send(invite.clone().into(), Some(&server_addr))
+        .await
+        .expect("send re-INVITE");
+
+    let mut incoming = endpoint.incoming_transactions().expect("incoming");
+    let mut tx = timeout(Duration::from_secs(2), incoming.recv())
+        .await
+        .expect("timeout waiting for incoming transaction")
+        .expect("no incoming transaction");
+    assert_eq!(tx.original.method, Method::Invite);
+
+    tx.reply(StatusCode::OK).await.expect("reply 200");
+    assert_eq!(tx.state, TransactionState::Completed);
+    assert!(tx.timer_g.is_some(), "Timer G must run for 2xx over UDP");
+
+    let mut buf = vec![0u8; 4096];
+    let (len, _) = timeout(Duration::from_secs(2), client_conn.recv_raw(&mut buf))
+        .await
+        .expect("timeout waiting for 200")
+        .expect("recv failed");
+    assert!(String::from_utf8_lossy(&buf[..len]).starts_with("SIP/2.0 200"));
+
+    // Delayed ACK of the previous re-INVITE (CSeq 1) on the same dialog.
+    client_conn_sip
+        .send(make_2xx_ack(&invite, 1).into(), Some(&server_addr))
+        .await
+        .expect("send stale ACK");
+
+    // Drive the transaction past the first Timer G expiry (T1 = 500ms).
+    let received = timeout(Duration::from_millis(1200), tx.receive()).await;
+    assert!(
+        received.is_err(),
+        "an ACK with CSeq 1 must not be delivered by the CSeq 2 INVITE transaction, got {:?}",
+        received.ok().flatten().map(|m| m.to_string())
+    );
+    assert_eq!(
+        tx.state,
+        TransactionState::Completed,
+        "an ACK with CSeq 1 must not confirm the CSeq 2 INVITE transaction"
+    );
+    assert!(tx.timer_g.is_some(), "Timer G must keep running");
+    assert_eq!(
+        endpoint.inner.waiting_ack.len(),
+        1,
+        "the dialog must still route ACKs to the CSeq 2 transaction"
+    );
+
+    // The 2xx was retransmitted by Timer G.
+    let (len, _) = timeout(Duration::from_secs(1), client_conn.recv_raw(&mut buf))
+        .await
+        .expect("timeout waiting for 200 retransmission")
+        .expect("recv failed");
+    assert!(String::from_utf8_lossy(&buf[..len]).starts_with("SIP/2.0 200"));
+
+    // The ACK with the matching CSeq confirms the transaction.
+    client_conn_sip
+        .send(make_2xx_ack(&invite, 2).into(), Some(&server_addr))
+        .await
+        .expect("send ACK");
+
+    let msg = timeout(Duration::from_secs(2), tx.receive())
+        .await
+        .expect("timeout waiting for ACK")
+        .expect("transaction ended before ACK");
+    match msg {
+        SipMessage::Request(req) => {
+            assert_eq!(req.method, Method::Ack);
+            assert_eq!(req.cseq_header().unwrap().seq().unwrap(), 2);
+        }
+        other => panic!("expected ACK, got {}", other),
+    }
+    assert_eq!(tx.state, TransactionState::Confirmed);
+    assert!(tx.timer_g.is_none(), "Timer G must stop once ACKed");
+    assert_eq!(endpoint.inner.waiting_ack.len(), 0);
+
+    token.cancel();
+    serve_handle.abort();
+}
+
+/// The same over TCP: a delayed ACK must neither confirm the transaction nor
+/// reach the TU, and the dialog must keep routing ACKs to the current INVITE.
+#[tokio::test]
+async fn test_server_invite_ignores_ack_with_other_cseq_over_tcp() {
+    use crate::transport::tcp_listener::TcpListenerConnection;
+    use tokio::io::AsyncWriteExt;
+    let token = CancellationToken::new();
+    let port = std::net::TcpListener::bind("127.0.0.1:0")
+        .unwrap()
+        .local_addr()
+        .unwrap()
+        .port();
+    let server_addr: std::net::SocketAddr = format!("127.0.0.1:{}", port).parse().unwrap();
+    let tl = TransportLayer::new(token.child_token());
+    tl.add_transport(
+        TcpListenerConnection::new(server_addr, None)
+            .await
+            .unwrap()
+            .into(),
+    );
+    let endpoint = EndpointBuilder::new()
+        .with_user_agent("rsipstack-test")
+        .with_transport_layer(tl)
+        .build();
+    endpoint
+        .inner
+        .transport_layer
+        .serve_listens()
+        .await
+        .unwrap();
+    let endpoint_inner = endpoint.inner.clone();
+    let serve_handle = tokio::spawn(async move {
+        let _ = endpoint_inner.serve().await;
+    });
+    let mut incoming = endpoint.incoming_transactions().expect("incoming");
+    let mut stream = tokio::net::TcpStream::connect(server_addr).await.unwrap();
+
+    let invite = make_reinvite("TCP", 2);
+    stream
+        .write_all(invite.to_string().as_bytes())
+        .await
+        .unwrap();
+    let mut tx = timeout(Duration::from_secs(2), incoming.recv())
+        .await
+        .expect("timeout waiting for incoming transaction")
+        .expect("no incoming transaction");
+    tx.reply(StatusCode::OK).await.expect("reply 200");
+    assert_eq!(tx.state, TransactionState::Completed);
+
+    // Delayed ACK of CSeq 1, then the ACK of CSeq 2, on the ordered stream:
+    // the first ACK the transaction delivers must be the CSeq 2 one.
+    let acks = make_2xx_ack(&invite, 1).to_string() + &make_2xx_ack(&invite, 2).to_string();
+    stream.write_all(acks.as_bytes()).await.unwrap();
+    let msg = timeout(Duration::from_secs(2), tx.receive())
+        .await
+        .expect("timeout waiting for ACK")
+        .expect("transaction ended before ACK");
+    match msg {
+        SipMessage::Request(req) => {
+            assert_eq!(req.method, Method::Ack);
+            assert_eq!(req.cseq_header().unwrap().seq().unwrap(), 2);
+        }
+        other => panic!("expected ACK, got {}", other),
+    }
+    assert_eq!(tx.state, TransactionState::Confirmed);
+    assert_eq!(endpoint.inner.waiting_ack.len(), 0);
+
+    token.cancel();
+    serve_handle.abort();
+}
diff --git a/src/transaction/transaction.rs b/src/transaction/transaction.rs
index 1bf01f5e..94ec14e6 100644
--- a/src/transaction/transaction.rs
+++ b/src/transaction/transaction.rs
@@ -725,6 +725,22 @@ impl Transaction {
             TransactionState::Completed | TransactionState::Confirmed
                 if req.method == Method::Ack =>
             {
+                // RFC 3261 §17.1.1.3 / §13.2.2.4: an ACK carries the CSeq
+                // number of the INVITE it acknowledges. ACKs for 2xx are
+                // routed per dialog (`waiting_ack`), so a delayed ACK of an
+                // earlier re-INVITE can land here; it must not confirm this
+                // transaction or stop its 2xx retransmissions.
+                let ack_seq = req.cseq_header().and_then(|c| c.seq()).ok();
+                let invite_seq = self.original.cseq_header().and_then(|c| c.seq()).ok();
+                if ack_seq != invite_seq {
+                    debug!(
+                        key = %self.key,
+                        ?ack_seq,
+                        ?invite_seq,
+                        "ignoring ACK with a CSeq that does not match the INVITE"
+                    );
+                    return None;
+                }
                 self.transition(TransactionState::Confirmed).ok();
                 return Some(req.into());
             }