diff --git a/moto/ec2/models/fleets.py b/moto/ec2/models/fleets.py
index 1fd2accfe45d..8afbdd5590da 100644
--- a/moto/ec2/models/fleets.py
+++ b/moto/ec2/models/fleets.py
@@ -171,31 +171,31 @@ def instances(self) -> Optional[list[dict[str, Any]]]:
         """
         Return instances for instant fleets, None for other fleet types.
         This is part of the CreateFleet response for instant fleets only.
+        AWS groups instances that share the same launch spec and lifecycle into
+        a single entry with aggregated InstanceIds.
         """
         if self.fleet_type != "instant":
             return None
 
-        instances = []
-
-        # Process on-demand instances
-        for item in self.on_demand_instances:
-            instance_data = self._build_instance_data(
-                instance=item["instance"],
-                lifecycle="on-demand",
-                launch_spec=item.get("launch_spec"),
-            )
-            instances.append(instance_data)
-
-        # Process spot instances
-        for spot_request in self.spot_requests:
-            instance_data = self._build_instance_data(
-                instance=spot_request.instance,
-                lifecycle="spot",
-                launch_spec=spot_request.launch_spec,
-            )
-            instances.append(instance_data)
+        # Flatten all launched instances into (lifecycle, launch_spec, instance) tuples.
+        # SpotFleetLaunchSpec is hashable and compares by content, so identical configs
+        # collapse into one group entry, matching AWS behaviour.
+        all_items: list[tuple[str, Optional[SpotFleetLaunchSpec], Any]] = [
+            ("on-demand", item.get("launch_spec"), item["instance"])
+            for item in self.on_demand_instances
+        ] + [("spot", req.launch_spec, req.instance) for req in self.spot_requests]
+
+        groups: dict[tuple[str, Optional[SpotFleetLaunchSpec]], dict[str, Any]] = {}
+        for lifecycle, launch_spec, instance in all_items:
+            key = (lifecycle, launch_spec)
+            if key not in groups:
+                groups[key] = self._build_instance_data(
+                    instance, lifecycle, launch_spec
+                )
+            else:
+                groups[key]["InstanceIds"].append(instance.id)
 
-        return instances
+        return list(groups.values())
 
     @staticmethod
     def _build_instance_data(
diff --git a/tests/test_ec2/test_fleets.py b/tests/test_ec2/test_fleets.py
index 914993aaff05..fd4bc3d77870 100644
--- a/tests/test_ec2/test_fleets.py
+++ b/tests/test_ec2/test_fleets.py
@@ -833,18 +833,25 @@ def test_create_fleet_api():
     assert fleet_res["FleetId"].startswith("fleet-") is True
 
     assert "Instances" in fleet_res
-    assert len(fleet_res["Instances"]) == 3
 
-    instance_ids = [i["InstanceIds"] for i in fleet_res["Instances"]]
-    for instance_id in instance_ids:
-        assert instance_id[0].startswith("i-") is True
+    instances = fleet_res["Instances"]
 
-    instance_types = [i["InstanceType"] for i in fleet_res["Instances"]]
-    assert instance_types == ["t2.micro", "t2.micro", "t2.micro"]
+    # AWS groups instances with the same config (lifecycle/template) into one entry;
+    # with 1 on-demand + 2 spot we expect 2 groups
+    assert len(instances) == 2
+
+    # Total instance IDs across all groups should equal the requested capacity
+    all_instance_ids = [iid for i in instances for iid in i["InstanceIds"]]
+    assert len(all_instance_ids) == 3
+    for instance_id in all_instance_ids:
+        assert instance_id.startswith("i-") is True
+
+    all_instance_types = {i["InstanceType"] for i in instances}
+    assert all_instance_types == {"t2.micro"}
 
-    lifecycle = [i["Lifecycle"] for i in fleet_res["Instances"]]
-    assert "spot" in lifecycle
-    assert "on-demand" in lifecycle
+    lifecycles = {i["Lifecycle"] for i in instances}
+    assert "spot" in lifecycles
+    assert "on-demand" in lifecycles
 
 
 @mock_aws