diff --git a/src/Unread.ts b/src/Unread.ts
index 7b86a013a595..15f8d5961046 100644
--- a/src/Unread.ts
+++ b/src/Unread.ts
@@ -72,6 +72,11 @@ export function doesRoomHaveUnreadMessages(room: Room): boolean {
 }
 
 export function doesRoomOrThreadHaveUnreadMessages(roomOrThread: Room | Thread): boolean {
+    // NOTE: this shares logic with hasUserReadEvent in
+    // matrix-js-sdk/src/models/read-receipt.ts. They are not combined (yet)
+    // because hasUserReadEvent is focussed on a single event, and this is
+    // focussed on the whole room/thread.
+
     // If there are no messages yet in the timeline then it isn't fully initialised
     // and cannot be unread.
     if (!roomOrThread || roomOrThread.timeline.length === 0) {
@@ -94,26 +99,52 @@ export function doesRoomOrThreadHaveUnreadMessages(roomOrThread: Room | Thread):
     // N.B. this is NOT a read marker (RM, aka "read up to marker"),
     // despite the name of the method :((
     const readUpToId = roomOrThread.getEventReadUpTo(myUserId!);
-
-    // this just looks at whatever history we have, which if we've only just started
-    // up probably won't be very much, so if the last couple of events are ones that
-    // don't count, we don't know if there are any events that do count between where
-    // we have and the read receipt. We could fetch more history to try & find out,
-    // but currently we just guess.
-
-    // Loop through messages, starting with the most recent...
-    for (let i = roomOrThread.timeline.length - 1; i >= 0; --i) {
-        const ev = roomOrThread.timeline[i];
-
-        if (ev.getId() == readUpToId) {
-            // If we've read up to this event, there's nothing more recent
-            // that counts and we can stop looking because the user's read
-            // this and everything before.
-            return false;
-        } else if (!shouldHideEvent(ev) && eventTriggersUnreadCount(ev)) {
-            // We've found a message that counts before we hit
-            // the user's read receipt, so this room is definitely unread.
-            return true;
+    const readEvent = roomOrThread.findEventById(readUpToId);
+
+    // If the event we have a receipt for is loaded, it's easy - we just check
+    // whether there are any events after it
+    if (readEvent) {
+        // this just looks at whatever history we have, which if we've only just started
+        // up probably won't be very much, so if the last couple of events are ones that
+        // don't count, we don't know if there are any events that do count between where
+        // we have and the read receipt. We could fetch more history to try & find out,
+        // but currently we just guess.
+
+        // Loop through messages, starting with the most recent...
+        for (let i = roomOrThread.timeline.length - 1; i >= 0; --i) {
+            const ev = roomOrThread.timeline[i];
+
+            if (ev.getId() == readUpToId) {
+                // If we've read up to this event, there's nothing more recent
+                // that counts and we can stop looking because the user's read
+                // this and everything before.
+                return false;
+            } else if (!shouldHideEvent(ev) && eventTriggersUnreadCount(ev)) {
+                // We've found a message that counts before we hit
+                // the user's read receipt, so this room is definitely unread.
+                return true;
+            }
+        }
+    } else {
+        // Guess based on the timestamp of the receipt
+
+        const receipt = roomOrThread.getReadReceiptForUserId(myUserId);
+        if (receipt) {
+            const receiptTimestamp = receipt.data.ts;
+            for (let i = roomOrThread.timeline.length - 1; i >= 0; --i) {
+                const ev = roomOrThread.timeline[i];
+
+                if (ev.getTs() < receiptTimestamp) {
+                    // If we've read up to this event, there's nothing more recent
+                    // that counts and we can stop looking because the user's read
+                    // this and everything before.
+                    return false;
+                } else if (!shouldHideEvent(ev) && eventTriggersUnreadCount(ev)) {
+                    // We've found a message that counts before we hit
+                    // the user's read receipt, so this room is definitely unread.
+                    return true;
+                }
+            }
         }
     }