diff --git a/libp2p/pubsub/pubsub.py b/libp2p/pubsub/pubsub.py
index 40c4036..2109a9c 100644
--- a/libp2p/pubsub/pubsub.py
+++ b/libp2p/pubsub/pubsub.py
@@ -1,4 +1,6 @@
+# pylint: disable=no-name-in-module
 import asyncio
+from lru import LRU
 
 from .pb import rpc_pb2
 from .pubsub_notifee import PubsubNotifee
@@ -7,7 +9,7 @@ from .pubsub_notifee import PubsubNotifee
 class Pubsub():
     # pylint: disable=too-many-instance-attributes, no-member
 
-    def __init__(self, host, router, my_id):
+    def __init__(self, host, router, my_id, cache_size=None):
         """
         Construct a new Pubsub object, which is responsible for handling all
         Pubsub-related messages and relaying messages as appropriate to the
@@ -37,8 +39,12 @@ class Pubsub():
         self.incoming_msgs_from_peers = asyncio.Queue()
         self.outgoing_messages = asyncio.Queue()
 
-        # TODO: Make seen_messages a cache (LRU cache?)
-        self.seen_messages = []
+        # keeps track of seen messages as LRU cache
+        if cache_size is None:
+            self.cache_size = 128
+        else:
+            self.cache_size = cache_size
+        self.seen_messages = LRU(self.cache_size)
 
         # Map of topics we are subscribed to to handler functions
         # for when the given topic receives a message
@@ -89,7 +95,7 @@ class Pubsub():
                     id_in_seen_msgs = (message.seqno, message.from_id)
                     if id_in_seen_msgs not in self.seen_messages:
                         should_publish = True
-                        self.seen_messages.append(id_in_seen_msgs)
+                        self.seen_messages[id_in_seen_msgs] = 1
                         await self.handle_talk(message)
 
             if rpc_incoming.subscriptions:
diff --git a/requirements_dev.txt b/requirements_dev.txt
index e3d8c0e..3d5ca3c 100644
--- a/requirements_dev.txt
+++ b/requirements_dev.txt
@@ -5,3 +5,4 @@ pytest-asyncio
 pylint
 grpcio
 grpcio-tools
+lru-dict>=1.1.6
\ No newline at end of file
diff --git a/setup.py b/setup.py
index c44a276..0c62145 100644
--- a/setup.py
+++ b/setup.py
@@ -23,7 +23,8 @@ setuptools.setup(
         "pymultihash",
         "multiaddr",
         "grpcio",
-        "grpcio-tools"
+        "grpcio-tools",
+        "lru-dict>=1.1.6"
     ],
     packages=["libp2p"],
     zip_safe=False,
diff --git a/tests/pubsub/test_floodsub.py b/tests/pubsub/test_floodsub.py
index 69b2f8c..06c1cbd 100644
--- a/tests/pubsub/test_floodsub.py
+++ b/tests/pubsub/test_floodsub.py
@@ -21,7 +21,7 @@ async def connect(node1, node2):
     await node1.connect(info)
 
 @pytest.mark.asyncio
-async def test_simple_two_nodes_RPC():
+async def test_simple_two_nodes():
     node_a = await new_node(transport_opt=["/ip4/127.0.0.1/tcp/0"])
     node_b = await new_node(transport_opt=["/ip4/127.0.0.1/tcp/0"])
 
@@ -58,6 +58,80 @@ async def test_simple_two_nodes_RPC():
     # Success, terminate pending tasks.
     await cleanup()
 
+@pytest.mark.asyncio
+async def test_lru_cache_two_nodes():
+    # two nodes with cache_size of 4
+    # node_a send the following messages to node_b
+    # [1, 1, 2, 1, 3, 1, 4, 1, 5, 1]
+    # node_b should only receive the following
+    # [1, 2, 3, 4, 5, 1]
+    node_a = await new_node(transport_opt=["/ip4/127.0.0.1/tcp/0"])
+    node_b = await new_node(transport_opt=["/ip4/127.0.0.1/tcp/0"])
+
+    await node_a.get_network().listen(multiaddr.Multiaddr("/ip4/127.0.0.1/tcp/0"))
+    await node_b.get_network().listen(multiaddr.Multiaddr("/ip4/127.0.0.1/tcp/0"))
+
+    supported_protocols = ["/floodsub/1.0.0"]
+
+    # initialize PubSub with a cache_size of 4
+    floodsub_a = FloodSub(supported_protocols)
+    pubsub_a = Pubsub(node_a, floodsub_a, "a", 4)
+    floodsub_b = FloodSub(supported_protocols)
+    pubsub_b = Pubsub(node_b, floodsub_b, "b", 4)
+
+    await connect(node_a, node_b)
+
+    await asyncio.sleep(0.25)
+    qb = await pubsub_b.subscribe("my_topic")
+
+    await asyncio.sleep(0.25)
+
+    node_a_id = str(node_a.get_id())
+
+    # initialize message_id_generator
+    # store first message
+    next_msg_id_func = message_id_generator(0)
+    first_message = generate_RPC_packet(node_a_id, ["my_topic"], "some data 1", next_msg_id_func())
+
+    await floodsub_a.publish(node_a_id, first_message.SerializeToString())
+    await asyncio.sleep(0.25)
+    print (first_message)
+
+    messages = [first_message]
+    # for the next 5 messages
+    for i in range(2, 6):
+        # write first message     
+        await floodsub_a.publish(node_a_id, first_message.SerializeToString())
+        await asyncio.sleep(0.25)
+
+        # generate and write next message
+        msg = generate_RPC_packet(node_a_id, ["my_topic"], "some data " + str(i), next_msg_id_func())
+        messages.append(msg)
+
+        await floodsub_a.publish(node_a_id, msg.SerializeToString())
+        await asyncio.sleep(0.25)
+
+    # write first message again
+    await floodsub_a.publish(node_a_id, first_message.SerializeToString())
+    await asyncio.sleep(0.25)
+
+    # check the first five messages in queue
+    # should only see 1 first_message
+    for i in range(5):
+        # Check that the msg received by node_b is the same
+        # as the message sent by node_a
+        res_b = await qb.get()
+        assert res_b.SerializeToString() == messages[i].publish[0].SerializeToString()
+
+    # the 6th message should be first_message
+    res_b = await qb.get()
+    assert res_b.SerializeToString() == first_message.publish[0].SerializeToString()
+    assert qb.empty()
+    
+    # Success, terminate pending tasks.
+    await cleanup()
+
+
 async def perform_test_from_obj(obj):
     """
     Perform a floodsub test from a test obj.