diff --git a/src/httpcore2/httpcore2/_async/http_proxy.py b/src/httpcore2/httpcore2/_async/http_proxy.py
index 5cde1dd4..b3c8ddac 100644
--- a/src/httpcore2/httpcore2/_async/http_proxy.py
+++ b/src/httpcore2/httpcore2/_async/http_proxy.py
@@ -294,9 +294,16 @@ async def handle_async_request(self, request: Request) -> Response:
                     "server_hostname": self._remote_origin.host.decode("ascii"),
                     "timeout": timeout,
                 }
-                async with Trace("start_tls", logger, request, kwargs) as trace:
-                    stream = await stream.start_tls(**kwargs)
-                    trace.return_value = stream
+                try:
+                    async with Trace("start_tls", logger, request, kwargs) as trace:
+                        stream = await stream.start_tls(**kwargs)
+                        trace.return_value = stream
+                except Exception:
+                    # If TLS setup fails, close the underlying CONNECT
+                    # connection so the pool can discard it instead of
+                    # leaving it ACTIVE until max_connections is exhausted.
+                    await self._connection.aclose()
+                    raise
 
                 # Determine if we should be using HTTP/1.1 or HTTP/2
                 ssl_object = stream.get_extra_info("ssl_object")
diff --git a/src/httpcore2/httpcore2/_sync/http_proxy.py b/src/httpcore2/httpcore2/_sync/http_proxy.py
index c0129f1a..46107bb9 100644
--- a/src/httpcore2/httpcore2/_sync/http_proxy.py
+++ b/src/httpcore2/httpcore2/_sync/http_proxy.py
@@ -294,9 +294,16 @@ def handle_request(self, request: Request) -> Response:
                     "server_hostname": self._remote_origin.host.decode("ascii"),
                     "timeout": timeout,
                 }
-                with Trace("start_tls", logger, request, kwargs) as trace:
-                    stream = stream.start_tls(**kwargs)
-                    trace.return_value = stream
+                try:
+                    with Trace("start_tls", logger, request, kwargs) as trace:
+                        stream = stream.start_tls(**kwargs)
+                        trace.return_value = stream
+                except Exception:
+                    # If TLS setup fails, close the underlying CONNECT
+                    # connection so the pool can discard it instead of
+                    # leaving it ACTIVE until max_connections is exhausted.
+                    self._connection.close()
+                    raise
 
                 # Determine if we should be using HTTP/1.1 or HTTP/2
                 ssl_object = stream.get_extra_info("ssl_object")
diff --git a/tests/httpcore2/_async/test_http_proxy.py b/tests/httpcore2/_async/test_http_proxy.py
index 4754eee1..dd30b67c 100644
--- a/tests/httpcore2/_async/test_http_proxy.py
+++ b/tests/httpcore2/_async/test_http_proxy.py
@@ -240,3 +240,48 @@ def test_proxy_headers() -> None:
         auth=("username", "password"),
     )
     assert proxy.headers == [(b"Proxy-Authorization", b"Basic dXNlcm5hbWU6cGFzc3dvcmQ=")]
+
+
+class BrokenTLSStream(AsyncMockStream):
+    async def start_tls(
+        self,
+        ssl_context: ssl.SSLContext,
+        server_hostname: typing.Optional[str] = None,
+        timeout: typing.Optional[float] = None,
+    ) -> AsyncNetworkStream:
+        raise OSError("TLS Failure")
+
+
+class BrokenTLSBackend(AsyncMockBackend):
+    async def connect_tcp(
+        self,
+        host: str,
+        port: int,
+        timeout: typing.Optional[float] = None,
+        local_address: typing.Optional[str] = None,
+        socket_options: typing.Optional[typing.Iterable[SOCKET_OPTION]] = None,
+    ) -> AsyncNetworkStream:
+        return BrokenTLSStream(list(self._buffer))
+
+
+@pytest.mark.anyio
+async def test_proxy_tunneling_tls_error() -> None:
+    """
+    Send an HTTPS request via a proxy where the TLS handshake fails after the
+    CONNECT tunnel is established. The CONNECT connection must be closed so it
+    doesn't leak from the pool.
+    """
+    network_backend = BrokenTLSBackend(
+        [
+            b"HTTP/1.1 200 OK\r\n\r\n",
+        ]
+    )
+
+    async with AsyncConnectionPool(
+        proxy=Proxy("http://localhost:8080/"),
+        network_backend=network_backend,
+    ) as proxy:
+        with pytest.raises(OSError, match="TLS Failure"):
+            await proxy.request("GET", "https://example.com/")
+
+        assert not proxy.connections
diff --git a/tests/httpcore2/_sync/test_http_proxy.py b/tests/httpcore2/_sync/test_http_proxy.py
index f183a1fe..2ef96180 100644
--- a/tests/httpcore2/_sync/test_http_proxy.py
+++ b/tests/httpcore2/_sync/test_http_proxy.py
@@ -240,3 +240,48 @@ def test_proxy_headers() -> None:
         auth=("username", "password"),
     )
     assert proxy.headers == [(b"Proxy-Authorization", b"Basic dXNlcm5hbWU6cGFzc3dvcmQ=")]
+
+
+class BrokenTLSStream(MockStream):
+    def start_tls(
+        self,
+        ssl_context: ssl.SSLContext,
+        server_hostname: typing.Optional[str] = None,
+        timeout: typing.Optional[float] = None,
+    ) -> NetworkStream:
+        raise OSError("TLS Failure")
+
+
+class BrokenTLSBackend(MockBackend):
+    def connect_tcp(
+        self,
+        host: str,
+        port: int,
+        timeout: typing.Optional[float] = None,
+        local_address: typing.Optional[str] = None,
+        socket_options: typing.Optional[typing.Iterable[SOCKET_OPTION]] = None,
+    ) -> NetworkStream:
+        return BrokenTLSStream(list(self._buffer))
+
+
+
+def test_proxy_tunneling_tls_error() -> None:
+    """
+    Send an HTTPS request via a proxy where the TLS handshake fails after the
+    CONNECT tunnel is established. The CONNECT connection must be closed so it
+    doesn't leak from the pool.
+    """
+    network_backend = BrokenTLSBackend(
+        [
+            b"HTTP/1.1 200 OK\r\n\r\n",
+        ]
+    )
+
+    with ConnectionPool(
+        proxy=Proxy("http://localhost:8080/"),
+        network_backend=network_backend,
+    ) as proxy:
+        with pytest.raises(OSError, match="TLS Failure"):
+            proxy.request("GET", "https://example.com/")
+
+        assert not proxy.connections