diff --git a/watchtower/wtwire/state_update_reply.go b/watchtower/wtwire/state_update_reply.go
new file mode 100644
index 00000000..b2258d0c
--- /dev/null
+++ b/watchtower/wtwire/state_update_reply.go
@@ -0,0 +1,86 @@
+package wtwire
+
+import "io"
+
+// StateUpdateCode is an error code returned by a watchtower in response to a
+// StateUpdate message.
+type StateUpdateCode = ErrorCode
+
+const (
+	// StateUpdateCodeClientBehind signals that the client's sequence number
+	// is behind what the watchtower expects based on its LastApplied. This
+	// error should cause the client to record the LastApplied field in the
+	// response, and initiate another attempt with the proper sequence
+	// number.
+	//
+	// NOTE: Repeated occurrences of this could be interpreted as an attempt
+	// to siphon state updates from the client. If the client believes it
+	// is not violating the protocol, this could be grounds to blacklist
+	// this tower from future session negotiation.
+	StateUpdateCodeClientBehind StateUpdateCode = 70
+
+	// StateUpdateCodeMaxUpdatesExceeded signals that the client tried to
+	// send a sequence number beyond the negotiated MaxUpdates of the
+	// session.
+	StateUpdateCodeMaxUpdatesExceeded StateUpdateCode = 71
+
+	// StateUpdateCodeSeqNumOutOfOrder signals the client sent an update
+	// that does not follow the required incremental monotonicity required
+	// by the tower.
+	StateUpdateCodeSeqNumOutOfOrder StateUpdateCode = 72
+)
+
+// StateUpdateReply is a message sent from watchtower to client in response to a
+// StateUpdate message, and signals either an acceptance or rejection of the
+// proposed state update.
+type StateUpdateReply struct {
+	// Code will be non-zero if the watchtower rejected the state update.
+	Code StateUpdateCode
+
+	// LastApplied returns the sequence number of the last accepted update
+	// known to the watchtower. If the update was successful, this value
+	// should be the sequence number of the last update sent.
+	LastApplied uint16
+}
+
+// A compile time check to ensure StateUpdateReply implements the wtwire.Message
+// interface.
+var _ Message = (*StateUpdateReply)(nil)
+
+// Decode deserializes a serialized StateUpdateReply message stored in the passed
+// io.Reader observing the specified protocol version.
+//
+// This is part of the wtwire.Message interface.
+func (t *StateUpdateReply) Decode(r io.Reader, pver uint32) error {
+	return ReadElements(r,
+		&t.Code,
+		&t.LastApplied,
+	)
+}
+
+// Encode serializes the target StateUpdateReply into the passed io.Writer
+// observing the protocol version specified.
+//
+// This is part of the wtwire.Message interface.
+func (t *StateUpdateReply) Encode(w io.Writer, pver uint32) error {
+	return WriteElements(w,
+		t.Code,
+		t.LastApplied,
+	)
+}
+
+// MsgType returns the integer uniquely identifying this message type on the
+// wire.
+//
+// This is part of the wtwire.Message interface.
+func (t *StateUpdateReply) MsgType() MessageType {
+	return MsgStateUpdateReply
+}
+
+// MaxPayloadLength returns the maximum allowed payload size for a
+// StateUpdateReply complete message observing the specified protocol version.
+//
+// This is part of the wtwire.Message interface.
+func (t *StateUpdateReply) MaxPayloadLength(uint32) uint32 {
+	return 4
+}