diff --git a/src/browser/Page.zig b/src/browser/Page.zig
index 5940d2c5..40b73618 100644
--- a/src/browser/Page.zig
+++ b/src/browser/Page.zig
@@ -1243,6 +1243,11 @@ pub fn notifyPerformanceObservers(self: *Page, entry: *Performance.Entry) !void
         }
     }
 
+    try self.schedulePerformanceObserverDelivery();
+}
+
+/// Schedules async delivery of performance observer records.
+pub fn schedulePerformanceObserverDelivery(self: *Page) !void {
     // Already scheduled.
     if (self._performance_delivery_scheduled) {
         return;
diff --git a/src/browser/tests/performance_observer/performance_observer.html b/src/browser/tests/performance_observer/performance_observer.html
index c1c546b7..3289ab41 100644
--- a/src/browser/tests/performance_observer/performance_observer.html
+++ b/src/browser/tests/performance_observer/performance_observer.html
@@ -68,3 +68,31 @@
 <script>
   testing.expectEqual(['mark', 'measure'], PerformanceObserver.supportedEntryTypes);
 </script>
+
+<script id="buffered_option">
+{
+  // Clear marks from previous tests so we get a precise count
+  performance.clearMarks();
+
+  // Create marks BEFORE the observer
+  performance.mark("early1", { startTime: 1.0 });
+  performance.mark("early2", { startTime: 2.0 });
+
+  let receivedEntries = null;
+  const observer = new PerformanceObserver((list) => {
+    receivedEntries = list.getEntries();
+  });
+
+  // With buffered: true, existing marks should be delivered
+  observer.observe({ type: "mark", buffered: true });
+
+  testing.eventually(() => {
+    testing.expectEqual(true, receivedEntries !== null);
+    testing.expectEqual(2, receivedEntries.length);
+    testing.expectEqual("early1", receivedEntries[0].name);
+    testing.expectEqual("early2", receivedEntries[1].name);
+
+    observer.disconnect();
+  });
+}
+</script>
diff --git a/src/browser/webapi/PerformanceObserver.zig b/src/browser/webapi/PerformanceObserver.zig
index ad130c97..cfabaebb 100644
--- a/src/browser/webapi/PerformanceObserver.zig
+++ b/src/browser/webapi/PerformanceObserver.zig
@@ -113,6 +113,20 @@ pub fn observe(
 
     // Update interests.
     self._interests = interests;
+
+    // Deliver existing entries if buffered option is set.
+    // Per spec, buffered is only valid with the type option, not entryTypes.
+    // Delivery is async via a queued task, not synchronous.
+    if (options.buffered and options.type != null and !self.hasRecords()) {
+        for (page.window._performance._entries.items) |entry| {
+            if (self.interested(entry)) {
+                try self._entries.append(page.arena, entry);
+            }
+        }
+        if (self.hasRecords()) {
+            try page.schedulePerformanceObserverDelivery();
+        }
+    }
 }
 
 pub fn disconnect(self: *PerformanceObserver, page: *Page) void {