diff --git a/resend/http_client_httpx.py b/resend/http_client_httpx.py
index 32ea193..87df027 100644
--- a/resend/http_client_httpx.py
+++ b/resend/http_client_httpx.py
@@ -1,3 +1,4 @@
+import asyncio
 from typing import Any, Dict, List, Mapping, Optional, Tuple, Union
 
 import httpx
@@ -8,10 +9,35 @@
 class HTTPXClient(AsyncHTTPClient):
     """
     Async HTTP client implementation using the httpx library.
+
+    The client holds a single :class:`httpx.AsyncClient` so that the underlying
+    TCP connection (and its TLS handshake) is reused across requests.
+
+    The underlying client is created lazily on first use rather than in
+    ``__init__``, because an ``httpx.AsyncClient`` binds its connection pool to
+    the running event loop and this class is instantiated at import time, when
+    no loop is running. If the running loop changes (for example, a second
+    ``asyncio.run(...)`` call), a fresh client is created for the new loop.
+
+    Call :meth:`aclose` when the client is no longer needed.
     """
 
     def __init__(self, timeout: int = 30):
         self._timeout = timeout
+        self._client: Optional[httpx.AsyncClient] = None
+        self._loop: Optional[asyncio.AbstractEventLoop] = None
+
+    def _get_client(self) -> httpx.AsyncClient:
+        loop = asyncio.get_running_loop()
+
+        if self._client is None or self._client.is_closed or self._loop is not loop:
+            # A client bound to a previous loop cannot be awaited on this one,
+            # and its pooled connections died with that loop, so it is dropped
+            # rather than closed here.
+            self._client = httpx.AsyncClient(timeout=self._timeout)
+            self._loop = loop
+
+        return self._client
 
     async def request(
         self,
@@ -22,26 +48,34 @@ async def request(
         files: Optional[Dict[str, Any]] = None,
         data: Optional[Dict[str, str]] = None,
     ) -> Tuple[bytes, int, Mapping[str, str]]:
+        client = self._get_client()
+
         try:
-            async with httpx.AsyncClient(timeout=self._timeout) as client:
-                if files is not None:
-                    resp = await client.request(
-                        method=method,
-                        url=url,
-                        headers=headers,
-                        files=files,
-                        data=data,
-                    )
-                else:
-                    resp = await client.request(
-                        method=method,
-                        url=url,
-                        headers=headers,
-                        json=json if data is None else None,
-                        data=data,
-                    )
-                return resp.content, resp.status_code, resp.headers
+            if files is not None:
+                resp = await client.request(
+                    method=method,
+                    url=url,
+                    headers=headers,
+                    files=files,
+                    data=data,
+                )
+            else:
+                resp = await client.request(
+                    method=method,
+                    url=url,
+                    headers=headers,
+                    json=json if data is None else None,
+                    data=data,
+                )
+            return resp.content, resp.status_code, resp.headers
         except httpx.RequestError as e:
             # This gets caught by the async request.perform() method
             # and raises a ResendError with the error type "HttpClientError"
             raise RuntimeError(f"Request failed: {e}") from e
+
+    async def aclose(self) -> None:
+        """Close the underlying client and release pooled connections."""
+        if self._client is not None and not self._client.is_closed:
+            await self._client.aclose()
+        self._client = None
+        self._loop = None
diff --git a/resend/http_client_requests.py b/resend/http_client_requests.py
index 8caf308..83363fc 100644
--- a/resend/http_client_requests.py
+++ b/resend/http_client_requests.py
@@ -8,10 +8,16 @@
 class RequestsClient(HTTPClient):
     """
     This is the default HTTP client implementation using the requests library.
+
+    The client holds a single :class:`requests.Session` so that the underlying
+    TCP connection (and its TLS handshake) is reused across requests. Call
+    :meth:`close` when the client is no longer needed, or use it as a context
+    manager.
     """
 
     def __init__(self, timeout: int = 30):
         self._timeout = timeout
+        self._session = requests.Session()
 
     def request(
         self,
@@ -24,7 +30,7 @@ def request(
     ) -> Tuple[bytes, int, Mapping[str, str]]:
         try:
             if files is not None:
-                resp = requests.request(
+                resp = self._session.request(
                     method=method,
                     url=url,
                     headers=headers,
@@ -33,7 +39,7 @@ def request(
                     timeout=self._timeout,
                 )
             else:
-                resp = requests.request(
+                resp = self._session.request(
                     method=method,
                     url=url,
                     headers=headers,
@@ -46,3 +52,13 @@ def request(
             # This gets caught by the request.perform() method
             # and raises a ResendError with the error type "HttpClientError"
             raise RuntimeError(f"Request failed: {e}") from e
+
+    def close(self) -> None:
+        """Close the underlying session and release pooled connections."""
+        self._session.close()
+
+    def __enter__(self) -> "RequestsClient":
+        return self
+
+    def __exit__(self, *args: Any) -> None:
+        self.close()
diff --git a/tests/http_client_connection_reuse_test.py b/tests/http_client_connection_reuse_test.py
new file mode 100644
index 0000000..58cdc36
--- /dev/null
+++ b/tests/http_client_connection_reuse_test.py
@@ -0,0 +1,199 @@
+import asyncio
+import json
+import socketserver
+import threading
+from concurrent.futures import ThreadPoolExecutor
+from contextlib import contextmanager
+from http.server import BaseHTTPRequestHandler
+from typing import Any, Iterator, List, Tuple
+
+import resend
+from resend.http_client_httpx import HTTPXClient
+from resend.http_client_requests import RequestsClient
+
+
+class _ThreadedServer(socketserver.ThreadingTCPServer):
+    daemon_threads = True
+    allow_reuse_address = True
+
+
+@contextmanager
+def serve() -> Iterator[Tuple[str, List[int]]]:
+    """Run a local HTTP server and record how many TCP connections it accepts.
+
+    Yields the base URL and the connection log. One entry is appended per
+    accepted connection, so reuse shows up as a shorter log than the number of
+    requests made.
+    """
+    connections: List[int] = []
+    lock = threading.Lock()
+
+    class Handler(BaseHTTPRequestHandler):
+        protocol_version = "HTTP/1.1"
+
+        def setup(self) -> None:
+            # setup() runs once per accepted TCP connection, not per request.
+            with lock:
+                connections.append(1)
+            super().setup()
+
+        def do_POST(self) -> None:
+            length = int(self.headers.get("Content-Length", 0))
+            sent = json.loads(self.rfile.read(length))
+
+            # Echo the subject back so a response delivered to the wrong
+            # caller is detectable.
+            body = json.dumps({"id": sent["subject"]}).encode()
+            self.send_response(200)
+            self.send_header("Content-Type", "application/json")
+            self.send_header("Content-Length", str(len(body)))
+            self.end_headers()
+            self.wfile.write(body)
+
+        def log_message(self, format: str, *args: Any) -> None:
+            pass
+
+    httpd = _ThreadedServer(("127.0.0.1", 0), Handler)
+    threading.Thread(target=httpd.serve_forever, daemon=True).start()
+
+    try:
+        yield f"http://127.0.0.1:{httpd.server_address[1]}", connections
+    finally:
+        httpd.shutdown()
+        httpd.server_close()
+
+
+SEND_PARAMS: resend.Emails.SendParams = {
+    "from": "hello@example.com",
+    "to": ["world@example.com"],
+    "subject": "Hi!",
+    "html": "<b>hi</b>",
+}
+
+
+class TestSyncConnectionReuse:
+    def setup_method(self) -> None:
+        self._original_client = resend.default_http_client
+        self._original_url = resend.api_url
+        resend.api_key = "re_test"
+
+    def teardown_method(self) -> None:
+        resend.default_http_client = self._original_client
+        resend.api_url = self._original_url
+        resend.api_key = None
+
+    def test_reuses_a_single_connection(self) -> None:
+        with serve() as (url, connections):
+            resend.api_url = url
+            client = RequestsClient()
+            resend.default_http_client = client
+
+            try:
+                for _ in range(5):
+                    resend.Emails.send(SEND_PARAMS)
+            finally:
+                client.close()
+
+            assert len(connections) == 1
+
+    def test_close_is_idempotent(self) -> None:
+        client = RequestsClient()
+        client.close()
+        client.close()
+
+    def test_shared_session_keeps_responses_separate_across_threads(self) -> None:
+        """The session is shared, so each caller must still get its own response."""
+        with serve() as (url, _connections):
+            resend.api_url = url
+            client = RequestsClient()
+            resend.default_http_client = client
+
+            def send(index: int) -> Tuple[int, str]:
+                params: resend.Emails.SendParams = {
+                    "from": "hello@example.com",
+                    "to": ["world@example.com"],
+                    "subject": f"msg-{index}",
+                    "html": "<b>hi</b>",
+                }
+                return index, resend.Emails.send(params)["id"]
+
+            try:
+                with ThreadPoolExecutor(max_workers=8) as pool:
+                    results = list(pool.map(send, range(40)))
+            finally:
+                client.close()
+
+            assert all(sent_id == f"msg-{index}" for index, sent_id in results)
+
+    def test_works_as_a_context_manager(self) -> None:
+        with serve() as (url, connections):
+            resend.api_url = url
+
+            with RequestsClient() as client:
+                resend.default_http_client = client
+                for _ in range(3):
+                    resend.Emails.send(SEND_PARAMS)
+
+            assert len(connections) == 1
+
+
+class TestAsyncConnectionReuse:
+    def setup_method(self) -> None:
+        self._original_client = resend.default_async_http_client
+        self._original_url = resend.api_url
+        resend.api_key = "re_test"
+
+    def teardown_method(self) -> None:
+        resend.default_async_http_client = self._original_client
+        resend.api_url = self._original_url
+        resend.api_key = None
+
+    def test_reuses_a_single_connection(self) -> None:
+        with serve() as (url, connections):
+            resend.api_url = url
+            client = HTTPXClient()
+            resend.default_async_http_client = client
+
+            async def send_many() -> None:
+                try:
+                    for _ in range(5):
+                        await resend.Emails.send_async(SEND_PARAMS)
+                finally:
+                    await client.aclose()
+
+            asyncio.run(send_many())
+
+            assert len(connections) == 1
+
+    def test_recreates_the_client_when_the_event_loop_changes(self) -> None:
+        """A client cached from a closed loop must not be reused on a new one."""
+        with serve() as (url, connections):
+            resend.api_url = url
+            client = HTTPXClient()
+            resend.default_async_http_client = client
+
+            async def send_one() -> None:
+                await resend.Emails.send_async(SEND_PARAMS)
+
+            # Two separate loops. The second must not fail on the dead pool.
+            asyncio.run(send_one())
+            asyncio.run(send_one())
+
+            # One connection per loop, and no error raised.
+            assert len(connections) == 2
+
+    def test_aclose_allows_a_later_request(self) -> None:
+        with serve() as (url, connections):
+            resend.api_url = url
+            client = HTTPXClient()
+            resend.default_async_http_client = client
+
+            async def send_close_send() -> None:
+                await resend.Emails.send_async(SEND_PARAMS)
+                await client.aclose()
+                await resend.Emails.send_async(SEND_PARAMS)
+                await client.aclose()
+
+            asyncio.run(send_close_send())
+
+            assert len(connections) == 2
diff --git a/tests/request_test.py b/tests/request_test.py
index 56e2b3d..2e772b0 100644
--- a/tests/request_test.py
+++ b/tests/request_test.py
@@ -11,7 +11,7 @@
 
 
 class TestResendRequest(unittest.TestCase):
-    @patch("resend.http_client_requests.requests.request")
+    @patch("resend.http_client_requests.requests.Session.request")
     @patch("resend.api_key", new="test_key")
     def test_request_idempotency_key_is_set(self, mock_requests: MagicMock) -> None:
         mock_response = Mock()
@@ -41,7 +41,7 @@ def test_request_idempotency_key_is_set(self, mock_requests: MagicMock) -> None:
         self.assertEqual(headers["User-Agent"], f"resend-python:{get_version()}")
         self.assertEqual(headers["Idempotency-Key"], "abc-123")
 
-    @patch("resend.http_client_requests.requests.request")
+    @patch("resend.http_client_requests.requests.Session.request")
     @patch("resend.api_key", new="test_key")
     def test_request_idempotency_key_is_not_set(self, mock_requests: MagicMock) -> None:
         mock_response = Mock()
@@ -72,7 +72,7 @@ def test_request_idempotency_key_is_not_set(self, mock_requests: MagicMock) -> N
             "Idempotency-Key", headers, "Idempotency-Key should not be set"
         )
 
-    @patch("resend.http_client_requests.requests.request")
+    @patch("resend.http_client_requests.requests.Session.request")
     @patch("resend.api_key", new="test_key")
     def test_non_json_preserves_http_status_when_client_error(
         self, mock_requests: MagicMock
@@ -101,7 +101,7 @@ def test_non_json_preserves_http_status_when_client_error(
         self.assertIn("text/html", err.message)
         self.assertEqual(err.headers.get("retry-after"), "2")
 
-    @patch("resend.http_client_requests.requests.request")
+    @patch("resend.http_client_requests.requests.Session.request")
     @patch("resend.api_key", new="test_key")
     def test_non_json_preserves_http_status_when_server_error(
         self, mock_requests: MagicMock
@@ -125,7 +125,7 @@ def test_non_json_preserves_http_status_when_server_error(
         self.assertEqual(err.code, 503)
         self.assertEqual(err.error_type, "application_error")
 
-    @patch("resend.http_client_requests.requests.request")
+    @patch("resend.http_client_requests.requests.Session.request")
     @patch("resend.api_key", new="test_key")
     def test_non_json_falls_back_to_500_when_status_is_success(
         self, mock_requests: MagicMock
@@ -150,7 +150,7 @@ def test_non_json_falls_back_to_500_when_status_is_success(
         self.assertEqual(err.error_type, "application_error")
         self.assertIn("text/html", err.message)
 
-    @patch("resend.http_client_requests.requests.request")
+    @patch("resend.http_client_requests.requests.Session.request")
     @patch("resend.api_key", new="test_key")
     def test_invalid_json_preserves_http_status(self, mock_requests: MagicMock) -> None:
         mock_response = Mock()
@@ -173,7 +173,7 @@ def test_invalid_json_preserves_http_status(self, mock_requests: MagicMock) -> N
         self.assertEqual(err.error_type, "application_error")
         self.assertEqual(err.message, "Failed to decode JSON response")
 
-    @patch("resend.http_client_requests.requests.request")
+    @patch("resend.http_client_requests.requests.Session.request")
     @patch("resend.api_key", new="test_key")
     def test_json_error_uses_http_status_when_body_omits_status_code(
         self, mock_requests: MagicMock