diff --git a/gateway/gateway.go b/gateway/gateway.go
index 2874ef24..89dd7ae3 100644
--- a/gateway/gateway.go
+++ b/gateway/gateway.go
@@ -41,7 +41,8 @@ type BrMsgID struct {
 var flog *log.Entry
 
 const (
-	apiProtocol = "api"
+	apiProtocol       = "api"
+	threadReplyPrefix = "thread reply"
 )
 
 func New(cfg config.Gateway, r *Router) *Gateway {
@@ -288,6 +289,11 @@ func (gw *Gateway) handleMessage(msg config.Message, dest *bridge.Bridge) []*BrM
 			msg.Channel = originchannel
 		}
 
+		// Add prefix if reply message is being unthreaded.
+		if msg.ParentID != "" && canonicalParentMsgID == "" {
+			msg.Text = fmt.Sprintf("%s: %s", threadReplyPrefix, msg.Text)
+		}
+
 		msg.ParentID = gw.getDestMsgID(origmsg.Protocol+" "+canonicalParentMsgID, dest, channel)
 		if msg.ParentID == "" {
 			msg.ParentID = canonicalParentMsgID