diff --git a/CHANGELOG.md b/CHANGELOG.md
index a243091b1d..c3a0c1dabb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - `opentelemetry-instrumentation-httpx` Fix `RequestInfo`/`ResponseInfo` type hints
   ([#3105](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3105))
+- `opentelemetry-instrumentation-httpx` Add the `is_instrumented` flag to the transport
+  ([#3106](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3106))
 
 
 ## Version 1.29.0/0.50b0 (2024-12-11)
diff --git a/instrumentation/opentelemetry-instrumentation-httpx/src/opentelemetry/instrumentation/httpx/__init__.py b/instrumentation/opentelemetry-instrumentation-httpx/src/opentelemetry/instrumentation/httpx/__init__.py
index 27bb3d639d..4eb4202c36 100644
--- a/instrumentation/opentelemetry-instrumentation-httpx/src/opentelemetry/instrumentation/httpx/__init__.py
+++ b/instrumentation/opentelemetry-instrumentation-httpx/src/opentelemetry/instrumentation/httpx/__init__.py
@@ -404,6 +404,8 @@ class SyncOpenTelemetryTransport(httpx.BaseTransport):
             that is called right before the span ends
     """
 
+    _is_instrumented_by_opentelemetry = True
+
     def __init__(
         self,
         transport: httpx.BaseTransport,
@@ -529,6 +531,8 @@ class AsyncOpenTelemetryTransport(httpx.AsyncBaseTransport):
             that is called right before the span ends
     """
 
+    _is_instrumented_by_opentelemetry = True
+
     def __init__(
         self,
         transport: httpx.AsyncBaseTransport,
@@ -897,6 +901,10 @@ def instrument_client(
                 "Attempting to instrument Httpx client while already instrumented"
             )
             return
+        if getattr(
+            client._transport, "is_instrumented_by_opentelemetry", False
+        ):
+            return
 
         _OpenTelemetrySemanticConventionStability._initialize()
         sem_conv_opt_in_mode = _OpenTelemetrySemanticConventionStability._get_opentelemetry_stability_opt_in_mode(
@@ -948,6 +956,7 @@ def instrument_client(
                             response_hook=response_hook,
                         ),
                     )
+            client._transport._is_instrumented_by_opentelemetry = True
             client._is_instrumented_by_opentelemetry = True
         if hasattr(client._transport, "handle_async_request"):
             wrap_function_wrapper(
@@ -974,6 +983,7 @@ def instrument_client(
                             async_response_hook=async_response_hook,
                         ),
                     )
+            client._transport._is_instrumented_by_opentelemetry = True
             client._is_instrumented_by_opentelemetry = True
 
     @staticmethod
@@ -989,9 +999,11 @@ def uninstrument_client(
             unwrap(client._transport, "handle_request")
             for transport in client._mounts.values():
                 unwrap(transport, "handle_request")
+            client._transport._is_instrumented_by_opentelemetry = False
             client._is_instrumented_by_opentelemetry = False
         elif hasattr(client._transport, "handle_async_request"):
             unwrap(client._transport, "handle_async_request")
             for transport in client._mounts.values():
                 unwrap(transport, "handle_async_request")
+            client._transport._is_instrumented_by_opentelemetry = False
             client._is_instrumented_by_opentelemetry = False
diff --git a/instrumentation/opentelemetry-instrumentation-httpx/tests/test_httpx_integration.py b/instrumentation/opentelemetry-instrumentation-httpx/tests/test_httpx_integration.py
index 148fe27893..9383bd269a 100644
--- a/instrumentation/opentelemetry-instrumentation-httpx/tests/test_httpx_integration.py
+++ b/instrumentation/opentelemetry-instrumentation-httpx/tests/test_httpx_integration.py
@@ -14,6 +14,8 @@
 
 # pylint: disable=too-many-lines
 
+from __future__ import annotations
+
 import abc
 import asyncio
 import typing
@@ -593,10 +595,10 @@ def create_transport(
         @abc.abstractmethod
         def create_client(
             self,
-            transport: typing.Union[
-                SyncOpenTelemetryTransport, AsyncOpenTelemetryTransport, None
-            ] = None,
-            **kwargs,
+            transport: SyncOpenTelemetryTransport
+            | AsyncOpenTelemetryTransport
+            | None = None,
+            **kwargs: typing.Any,
         ):
             pass
 
@@ -730,9 +732,9 @@ class BaseInstrumentorTest(BaseTest, metaclass=abc.ABCMeta):
         @abc.abstractmethod
         def create_client(
             self,
-            transport: typing.Union[
-                SyncOpenTelemetryTransport, AsyncOpenTelemetryTransport, None
-            ] = None,
+            transport: SyncOpenTelemetryTransport
+            | AsyncOpenTelemetryTransport
+            | None = None,
             **kwargs,
         ):
             pass
@@ -926,6 +928,17 @@ def test_instrument_client_called_on_the_class(self):
             self.assertEqual(result.text, "Hello!")
             self.assert_span(num_spans=1)
 
+        def test_instrument_multiple_clients_with_the_same_transport(self):
+            client1 = self.create_client()
+            client2 = self.create_client(transport=client1._transport)
+
+            HTTPXClientInstrumentor().instrument_client(client1)
+            HTTPXClientInstrumentor().instrument_client(client2)
+
+            result = self.perform_request(self.URL, client=client1)
+            self.assertEqual(result.text, "Hello!")
+            self.assert_span(num_spans=1)
+
         def test_instrumentation_without_client(self):
             HTTPXClientInstrumentor().instrument()
             results = [
@@ -1210,7 +1223,7 @@ def create_client(
         transport: typing.Optional[SyncOpenTelemetryTransport] = None,
         **kwargs,
     ):
-        return httpx.Client(**kwargs)
+        return httpx.Client(transport=transport, **kwargs)
 
     def perform_request(
         self,
@@ -1260,7 +1273,7 @@ def create_client(
         transport: typing.Optional[AsyncOpenTelemetryTransport] = None,
         **kwargs,
     ):
-        return httpx.AsyncClient(**kwargs)
+        return httpx.AsyncClient(transport=transport, **kwargs)
 
     def perform_request(
         self,