diff --git a/test/asynchronous/test_async_network_layer.py b/test/asynchronous/test_async_network_layer.py
new file mode 100644
index 0000000000..2480bbf61c
--- /dev/null
+++ b/test/asynchronous/test_async_network_layer.py
@@ -0,0 +1,211 @@
+# Copyright 2026-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Async-only unit tests for network_layer.py."""
+
+from __future__ import annotations
+
+import asyncio
+import struct
+import sys
+from unittest.mock import AsyncMock, MagicMock, patch
+
+sys.path[0:0] = [""]
+
+from test.asynchronous import AsyncUnitTest, unittest
+
+from pymongo.common import MAX_MESSAGE_SIZE
+from pymongo.errors import ProtocolError
+from pymongo.network_layer import PyMongoProtocol, _async_socket_receive
+
+
+async def _make_protocol(timeout=None):
+    protocol = PyMongoProtocol(timeout=timeout)
+    mock_transport = MagicMock()
+    mock_transport.is_closing.return_value = False
+    protocol.transport = mock_transport
+    return protocol
+
+
+def _make_header(length, request_id, response_to, op_code):
+    return struct.pack("<iiii", length, request_id, response_to, op_code)
+
+
+class TestPyMongoProtocol(AsyncUnitTest):
+    async def _make_proto_with_header(self, header_bytes, max_size=MAX_MESSAGE_SIZE):
+        protocol = await _make_protocol()
+        protocol._max_message_size = max_size
+        protocol._header = memoryview(bytearray(header_bytes))
+        return protocol
+
+    async def test_initial_timeout_from_constructor(self):
+        protocol = await _make_protocol(timeout=3.0)
+        self.assertEqual(protocol.gettimeout, 3.0)
+
+    async def test_settimeout_updates_value(self):
+        protocol = await _make_protocol()
+        protocol.settimeout(7.5)
+        self.assertEqual(protocol.gettimeout, 7.5)
+
+    async def test_default_timeout_is_none(self):
+        protocol = await _make_protocol()
+        self.assertIsNone(protocol.gettimeout)
+
+    async def test_normal_op_msg(self):
+        header = _make_header(length=32, request_id=1, response_to=99, op_code=2013)
+        protocol = await self._make_proto_with_header(header)
+        body_len, op_code, response_to, expecting_compression = protocol.process_header()
+        self.assertEqual(body_len, 16)
+        self.assertEqual(op_code, 2013)
+        self.assertEqual(response_to, 99)
+        self.assertFalse(expecting_compression)
+
+    async def test_op_compressed(self):
+        # OP_COMPRESSED=2012; process_header strips the 9-byte compression sub-header
+        # (op code + uncompressed size + compressor id), then the 16-byte standard header.
+        # length=35 → after compression sub-header: 26 → body: 10
+        header = _make_header(length=35, request_id=1, response_to=0, op_code=2012)
+        protocol = await self._make_proto_with_header(header)
+        body_len, op_code, _response_to, expecting_compression = protocol.process_header()
+        self.assertEqual(body_len, 10)
+        self.assertEqual(op_code, 2012)
+        self.assertTrue(expecting_compression)
+
+    async def test_op_compressed_length_too_small_raises(self):
+        header = _make_header(length=25, request_id=1, response_to=0, op_code=2012)
+        protocol = await self._make_proto_with_header(header)
+        with self.assertRaises(ProtocolError):
+            protocol.process_header()
+
+    async def test_non_compressed_length_too_small_raises(self):
+        header = _make_header(length=16, request_id=1, response_to=0, op_code=2013)
+        protocol = await self._make_proto_with_header(header)
+        with self.assertRaises(ProtocolError):
+            protocol.process_header()
+
+    async def test_length_exceeds_max_raises(self):
+        header = _make_header(
+            length=MAX_MESSAGE_SIZE + 1, request_id=1, response_to=0, op_code=2013
+        )
+        protocol = await self._make_proto_with_header(header)
+        with self.assertRaises(ProtocolError):
+            protocol.process_header()
+
+    async def test_op_reply_op_code(self):
+        header = _make_header(length=20, request_id=0, response_to=0, op_code=1)
+        protocol = await self._make_proto_with_header(header)
+        body_len, op_code, _response_to, expecting_compression = protocol.process_header()
+        self.assertEqual(body_len, 4)
+        self.assertEqual(op_code, 1)
+        self.assertFalse(expecting_compression)
+
+    async def test_compression_header_snappy_compressor_id(self):
+        protocol = await _make_protocol()
+        # <iiB: little-endian, i32 op code=2013, i32 uncompressed size=0, u8 compressor id=1 (snappy)
+        data = struct.pack("<iiB", 2013, 0, 1)
+        protocol._compression_header = memoryview(bytearray(data))
+        op_code, compressor_id = protocol.process_compression_header()
+        self.assertEqual(op_code, 2013)
+        self.assertEqual(compressor_id, 1)
+
+    async def test_compression_header_zlib_compressor_id(self):
+        protocol = await _make_protocol()
+        data = struct.pack("<iiB", 2013, 0, 2)
+        protocol._compression_header = memoryview(bytearray(data))
+        _, compressor_id = protocol.process_compression_header()
+        self.assertEqual(compressor_id, 2)
+
+    async def test_message_complete_resolves_pending_future(self):
+        protocol = await _make_protocol()
+        protocol._expecting_header = False
+        protocol._expecting_compression = False
+        protocol._message_size = 10
+        protocol._message = memoryview(bytearray(10))
+        protocol._message_index = 0
+        protocol._op_code = 2013
+        protocol._compressor_id = None
+        protocol._response_to = 42
+
+        future = asyncio.get_running_loop().create_future()
+        protocol._pending_messages.append(future)
+
+        protocol.buffer_updated(10)
+        self.assertTrue(future.done())
+        op_code, compressor_id, response_to, _ = future.result()
+        self.assertEqual(op_code, 2013)
+        self.assertIsNone(compressor_id)
+        self.assertEqual(response_to, 42)
+
+    async def test_close_aborts_transport(self):
+        protocol = await _make_protocol()
+        protocol.close()
+        self.assertTrue(protocol.transport.abort.called)
+
+    async def test_connection_lost_twice_does_not_raise(self):
+        protocol = await _make_protocol()
+        protocol.connection_lost(None)
+        protocol.connection_lost(None)
+
+    async def test_close_with_exception_propagates_to_pending(self):
+        protocol = await _make_protocol()
+        future = asyncio.get_running_loop().create_future()
+        protocol._pending_messages.append(future)
+        exc = OSError("connection reset")
+        protocol.close(exc)
+        with self.assertRaisesRegex(OSError, "connection reset"):
+            await future
+
+
+class TestAsyncSocketReceive(AsyncUnitTest):
+    async def test_reads_data_in_multiple_chunks(self):
+        # Covers the loop in _async_socket_receive that accumulates short reads
+        # until the requested length has been received.
+        data = b"abcdefgh"
+        length = len(data)
+        chunk1, chunk2 = data[:4], data[4:]
+        mock_socket = MagicMock()
+        loop = asyncio.get_running_loop()
+        calls = 0
+
+        async def fake_recv_into(sock, buf):
+            nonlocal calls
+            if calls == 0:
+                buf[: len(chunk1)] = chunk1
+                calls += 1
+                return len(chunk1)
+            buf[: len(chunk2)] = chunk2
+            calls += 1
+            return len(chunk2)
+
+        with patch.object(loop, "sock_recv_into", new=AsyncMock(side_effect=fake_recv_into)):
+            result = await _async_socket_receive(mock_socket, length, loop)
+        self.assertEqual(bytes(result), data)
+        self.assertEqual(calls, 2)
+
+    async def test_raises_on_connection_closed(self):
+        # Covers the explicit `raise OSError("connection closed")` branch when
+        # sock_recv_into returns 0.
+        mock_socket = MagicMock()
+        loop = asyncio.get_running_loop()
+
+        async def fake_recv_into(sock, buf):
+            return 0
+
+        with patch.object(loop, "sock_recv_into", new=AsyncMock(side_effect=fake_recv_into)):
+            with self.assertRaisesRegex(OSError, "connection closed"):
+                await _async_socket_receive(mock_socket, 10, loop)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/test/asynchronous/test_network_layer.py b/test/asynchronous/test_network_layer.py
new file mode 100644
index 0000000000..4b6511490b
--- /dev/null
+++ b/test/asynchronous/test_network_layer.py
@@ -0,0 +1,64 @@
+# Copyright 2026-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for code in network_layer.py shared between sync and async APIs.
+
+Async-only tests live in ``test_async_network_layer.py``.
+"""
+
+from __future__ import annotations
+
+import sys
+from unittest.mock import MagicMock
+
+sys.path[0:0] = [""]
+
+from test.asynchronous import AsyncUnitTest, unittest
+
+from pymongo.network_layer import NetworkingInterfaceBase
+
+_IS_SYNC = False
+
+
+class TestNetworkingInterfaceBase(AsyncUnitTest):
+    async def asyncSetUp(self):
+        self.base = NetworkingInterfaceBase(MagicMock())
+
+    def test_gettimeout_raises(self):
+        with self.assertRaises(NotImplementedError):
+            _ = self.base.gettimeout
+
+    def test_settimeout_raises(self):
+        with self.assertRaises(NotImplementedError):
+            self.base.settimeout(1.0)
+
+    def test_close_raises(self):
+        with self.assertRaises(NotImplementedError):
+            self.base.close()
+
+    def test_is_closing_raises(self):
+        with self.assertRaises(NotImplementedError):
+            self.base.is_closing()
+
+    def test_get_conn_raises(self):
+        with self.assertRaises(NotImplementedError):
+            _ = self.base.get_conn
+
+    def test_sock_raises(self):
+        with self.assertRaises(NotImplementedError):
+            _ = self.base.sock
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/test/test_network_layer.py b/test/test_network_layer.py
new file mode 100644
index 0000000000..8f3281de1c
--- /dev/null
+++ b/test/test_network_layer.py
@@ -0,0 +1,64 @@
+# Copyright 2026-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for code in network_layer.py shared between sync and async APIs.
+
+Async-only tests live in ``test_async_network_layer.py``.
+"""
+
+from __future__ import annotations
+
+import sys
+from unittest.mock import MagicMock
+
+sys.path[0:0] = [""]
+
+from test import UnitTest, unittest
+
+from pymongo.network_layer import NetworkingInterfaceBase
+
+_IS_SYNC = True
+
+
+class TestNetworkingInterfaceBase(UnitTest):
+    def setUp(self):
+        self.base = NetworkingInterfaceBase(MagicMock())
+
+    def test_gettimeout_raises(self):
+        with self.assertRaises(NotImplementedError):
+            _ = self.base.gettimeout
+
+    def test_settimeout_raises(self):
+        with self.assertRaises(NotImplementedError):
+            self.base.settimeout(1.0)
+
+    def test_close_raises(self):
+        with self.assertRaises(NotImplementedError):
+            self.base.close()
+
+    def test_is_closing_raises(self):
+        with self.assertRaises(NotImplementedError):
+            self.base.is_closing()
+
+    def test_get_conn_raises(self):
+        with self.assertRaises(NotImplementedError):
+            _ = self.base.get_conn
+
+    def test_sock_raises(self):
+        with self.assertRaises(NotImplementedError):
+            _ = self.base.sock
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/tools/synchro.py b/tools/synchro.py
index ed794c5963..850921d65d 100644
--- a/tools/synchro.py
+++ b/tools/synchro.py
@@ -128,6 +128,7 @@
     "SpecRunnerTask": "SpecRunnerThread",
     "AsyncMockConnection": "MockConnection",
     "AsyncMockPool": "MockPool",
+    "AsyncMock": "MagicMock",
     "StopAsyncIteration": "StopIteration",
     "create_async_event": "create_event",
     "async_create_barrier": "create_barrier",
@@ -190,6 +191,7 @@ def async_only_test(f: str) -> bool:
         "test_async_loop_safety.py",
         "test_async_contextvars_reset.py",
         "test_async_loop_unblocked.py",
+        "test_async_network_layer.py",
     ]
 
 
@@ -251,6 +253,7 @@ def async_only_test(f: str) -> bool:
     "test_monitor.py",
     "test_monitoring.py",
     "test_mongos_load_balancing.py",
+    "test_network_layer.py",
     "test_on_demand_csfle.py",
     "test_pooling.py",
     "test_raw_bson.py",