diff --git a/libp2p/pubsub/pubsub.py b/libp2p/pubsub/pubsub.py
index 493a303..fab8024 100644
--- a/libp2p/pubsub/pubsub.py
+++ b/libp2p/pubsub/pubsub.py
@@ -16,6 +16,7 @@ from typing import (
 import base58
 from lru import LRU
 
+from libp2p.crypto.keys import PrivateKey
 from libp2p.exceptions import ParseError, ValidationError
 from libp2p.host.host_interface import IHost
 from libp2p.io.exceptions import IncompleteReadError
@@ -28,7 +29,7 @@ from libp2p.utils import encode_varint_prefixed, read_varint_prefixed_bytes
 
 from .pb import rpc_pb2
 from .pubsub_notifee import PubsubNotifee
-from .validators import signature_validator
+from .validators import PUBSUB_SIGNING_PREFIX, signature_validator
 
 if TYPE_CHECKING:
     from .pubsub_router_interface import IPubsubRouter  # noqa: F401
@@ -82,8 +83,17 @@ class Pubsub:
 
     _tasks: List["asyncio.Future[Any]"]
 
+    # Indicate if we should enforce signature verification
+    strict_signing: bool
+    sign_key: PrivateKey
+
     def __init__(
-        self, host: IHost, router: "IPubsubRouter", my_id: ID, cache_size: int = None
+        self,
+        host: IHost,
+        router: "IPubsubRouter",
+        my_id: ID,
+        cache_size: int = None,
+        strict_signing: bool = True,
     ) -> None:
         """
         Construct a new Pubsub object, which is responsible for handling all
@@ -147,6 +157,12 @@ class Pubsub:
         self._tasks.append(asyncio.ensure_future(self.handle_peer_queue()))
         self._tasks.append(asyncio.ensure_future(self.handle_dead_peer_queue()))
 
+        self.strict_signing = strict_signing
+        if strict_signing:
+            self.sign_key = self.host.get_private_key()
+        else:
+            self.sign_key = None
+
     def get_hello_packet(self) -> rpc_pb2.RPC:
         """Generate subscription message with all topics we are subscribed to
         only send hello packet if we have subscribed topics."""
@@ -456,7 +472,13 @@ class Pubsub:
             seqno=self._next_seqno(),
         )
 
-        # TODO: Sign with our signing key
+        if self.strict_signing:
+            priv_key = self.sign_key
+            signature = priv_key.sign(
+                PUBSUB_SIGNING_PREFIX.encode() + msg.SerializeToString()
+            )
+            msg.key = self.host.get_public_key().serialize()
+            msg.signature = signature
 
         await self.push_msg(self.host.get_id(), msg)
 
@@ -505,18 +527,17 @@ class Pubsub:
 
         # TODO: Check if the `from` is in the blacklist. If yes, reject.
 
-        # TODO: Check if signing is required and if so signature should be attached.
-
         # If the message is processed before, return(i.e., don't further process the message).
         if self._is_msg_seen(msg):
             return
 
-        # TODO: - Validate the message. If failed, reject it.
-        # Validate the signature of the message
-        # FIXME: `signature_validator` is currently a stub.
-        if not signature_validator(msg.key, msg.SerializeToString()):
-            logger.debug("Signature validation failed for msg: %s", msg)
-            return
+        # Check if signing is required and if so validate the signature
+        if self.strict_signing:
+            # Validate the signature of the message
+            if not signature_validator(msg):
+                logger.debug("Signature validation failed for msg: %s", msg)
+                return
+
         # Validate the message with registered topic validators.
         # If the validation failed, return(i.e., don't further process the message).
         try:
diff --git a/libp2p/pubsub/validators.py b/libp2p/pubsub/validators.py
index d197207..22f6579 100644
--- a/libp2p/pubsub/validators.py
+++ b/libp2p/pubsub/validators.py
@@ -1,10 +1,41 @@
-# FIXME: Replace the type of `pubkey` with a custom type `Pubkey`
-def signature_validator(pubkey: bytes, msg: bytes) -> bool:
+import logging
+
+from libp2p.crypto.serialization import deserialize_public_key
+from libp2p.peer.id import ID
+
+from .pb import rpc_pb2
+
+logger = logging.getLogger("libp2p.pubsub")
+
+PUBSUB_SIGNING_PREFIX = "libp2p-pubsub:"
+
+
+def signature_validator(msg: rpc_pb2.Message) -> bool:
     """
     Verify the message against the given public key.
 
     :param pubkey: the public key which signs the message.
     :param msg: the message signed.
     """
-    # TODO: Implement the signature validation
-    return True
+    # Check if signature is attached
+    if msg.signature == b"":
+        logger.debug("Reject because no signature attached for msg: %s", msg)
+        return False
+
+    # Validate if message sender matches message signer,
+    # i.e., check if `msg.key` matches `msg.from_id`
+    msg_pubkey = deserialize_public_key(msg.key)
+    if ID.from_pubkey(msg_pubkey) != msg.from_id:
+        logger.debug(
+            "Reject because signing key does not match sender ID for msg: %s", msg
+        )
+        return False
+    # First, construct the original payload that's signed by 'msg.key'
+    msg_without_key_sig = rpc_pb2.Message(
+        data=msg.data, topicIDs=msg.topicIDs, from_id=msg.from_id, seqno=msg.seqno
+    )
+    payload = PUBSUB_SIGNING_PREFIX.encode() + msg_without_key_sig.SerializeToString()
+    try:
+        return msg_pubkey.verify(payload, msg.signature)
+    except Exception:
+        return False
diff --git a/libp2p/tools/factories.py b/libp2p/tools/factories.py
index b5c16b8..b189cfa 100644
--- a/libp2p/tools/factories.py
+++ b/libp2p/tools/factories.py
@@ -153,6 +153,7 @@ class PubsubFactory(factory.Factory):
     router = None
     my_id = factory.LazyAttribute(lambda obj: obj.host.get_id())
     cache_size = None
+    strict_signing = False
 
 
 async def swarm_pair_factory(
diff --git a/tests/pubsub/conftest.py b/tests/pubsub/conftest.py
index 9dbe90b..520fdf4 100644
--- a/tests/pubsub/conftest.py
+++ b/tests/pubsub/conftest.py
@@ -4,14 +4,24 @@ from libp2p.tools.constants import GOSSIPSUB_PARAMS
 from libp2p.tools.factories import FloodsubFactory, GossipsubFactory, PubsubFactory
 
 
-def _make_pubsubs(hosts, pubsub_routers, cache_size):
+@pytest.fixture
+def is_strict_signing():
+    return False
+
+
+def _make_pubsubs(hosts, pubsub_routers, cache_size, is_strict_signing):
     if len(pubsub_routers) != len(hosts):
         raise ValueError(
             f"lenght of pubsub_routers={pubsub_routers} should be equaled to the "
             f"length of hosts={len(hosts)}"
         )
     return tuple(
-        PubsubFactory(host=host, router=router, cache_size=cache_size)
+        PubsubFactory(
+            host=host,
+            router=router,
+            cache_size=cache_size,
+            strict_signing=is_strict_signing,
+        )
         for host, router in zip(hosts, pubsub_routers)
     )
 
@@ -27,16 +37,22 @@ def gossipsub_params():
 
 
 @pytest.fixture
-def pubsubs_fsub(num_hosts, hosts, pubsub_cache_size):
+def pubsubs_fsub(num_hosts, hosts, pubsub_cache_size, is_strict_signing):
     floodsubs = FloodsubFactory.create_batch(num_hosts)
-    _pubsubs_fsub = _make_pubsubs(hosts, floodsubs, pubsub_cache_size)
+    _pubsubs_fsub = _make_pubsubs(
+        hosts, floodsubs, pubsub_cache_size, is_strict_signing
+    )
     yield _pubsubs_fsub
     # TODO: Clean up
 
 
 @pytest.fixture
-def pubsubs_gsub(num_hosts, hosts, pubsub_cache_size, gossipsub_params):
+def pubsubs_gsub(
+    num_hosts, hosts, pubsub_cache_size, gossipsub_params, is_strict_signing
+):
     gossipsubs = GossipsubFactory.create_batch(num_hosts, **gossipsub_params._asdict())
-    _pubsubs_gsub = _make_pubsubs(hosts, gossipsubs, pubsub_cache_size)
+    _pubsubs_gsub = _make_pubsubs(
+        hosts, gossipsubs, pubsub_cache_size, is_strict_signing
+    )
     yield _pubsubs_gsub
     # TODO: Clean up
diff --git a/tests/pubsub/test_pubsub.py b/tests/pubsub/test_pubsub.py
index ebe2003..01d8ba7 100644
--- a/tests/pubsub/test_pubsub.py
+++ b/tests/pubsub/test_pubsub.py
@@ -6,6 +6,7 @@ import pytest
 from libp2p.exceptions import ValidationError
 from libp2p.peer.id import ID
 from libp2p.pubsub.pb import rpc_pb2
+from libp2p.pubsub.pubsub import PUBSUB_SIGNING_PREFIX
 from libp2p.tools.pubsub.utils import make_pubsub_msg
 from libp2p.tools.utils import connect
 from libp2p.utils import encode_varint_prefixed
@@ -510,3 +511,70 @@ async def test_push_msg(pubsubs_fsub, monkeypatch):
     await pubsubs_fsub[0].push_msg(pubsubs_fsub[0].my_id, msg_2)
     await asyncio.sleep(0.01)
     assert not event.is_set()
+
+
+@pytest.mark.parametrize("num_hosts, is_strict_signing", ((2, True),))
+@pytest.mark.asyncio
+async def test_strict_signing(pubsubs_fsub, hosts):
+    await connect(hosts[0], hosts[1])
+    await pubsubs_fsub[0].subscribe(TESTING_TOPIC)
+    await pubsubs_fsub[1].subscribe(TESTING_TOPIC)
+    await asyncio.sleep(1)
+
+    await pubsubs_fsub[0].publish(TESTING_TOPIC, TESTING_DATA)
+    await asyncio.sleep(1)
+
+    assert len(pubsubs_fsub[0].seen_messages) == 1
+    assert len(pubsubs_fsub[1].seen_messages) == 1
+
+
+@pytest.mark.parametrize("num_hosts, is_strict_signing", ((2, True),))
+@pytest.mark.asyncio
+async def test_strict_signing_failed_validation(pubsubs_fsub, hosts, monkeypatch):
+    msg = make_pubsub_msg(
+        origin_id=pubsubs_fsub[0].my_id,
+        topic_ids=[TESTING_TOPIC],
+        data=TESTING_DATA,
+        seqno=b"\x00" * 8,
+    )
+    priv_key = pubsubs_fsub[0].sign_key
+    signature = priv_key.sign(PUBSUB_SIGNING_PREFIX.encode() + msg.SerializeToString())
+
+    event = asyncio.Event()
+
+    def _is_msg_seen(msg):
+        return False
+
+    # Use router publish to check if `push_msg` succeed.
+    async def router_publish(*args, **kwargs):
+        # The event will only be set if `push_msg` succeed.
+        event.set()
+
+    monkeypatch.setattr(pubsubs_fsub[0], "_is_msg_seen", _is_msg_seen)
+    monkeypatch.setattr(pubsubs_fsub[0].router, "publish", router_publish)
+
+    # Test: no signature attached in `msg`
+    await pubsubs_fsub[0].push_msg(pubsubs_fsub[0].my_id, msg)
+    await asyncio.sleep(0.01)
+    assert not event.is_set()
+
+    # Test: `msg.key` does not match `msg.from_id`
+    msg.key = hosts[1].get_public_key().serialize()
+    msg.signature = signature
+    await pubsubs_fsub[0].push_msg(pubsubs_fsub[0].my_id, msg)
+    await asyncio.sleep(0.01)
+    assert not event.is_set()
+
+    # Test: invalid signature
+    msg.key = hosts[0].get_public_key().serialize()
+    msg.signature = b"\x12" * 100
+    await pubsubs_fsub[0].push_msg(pubsubs_fsub[0].my_id, msg)
+    await asyncio.sleep(0.01)
+    assert not event.is_set()
+
+    # Finally, assert the signature indeed will pass validation
+    msg.key = hosts[0].get_public_key().serialize()
+    msg.signature = signature
+    await pubsubs_fsub[0].push_msg(pubsubs_fsub[0].my_id, msg)
+    await asyncio.sleep(0.01)
+    assert event.is_set()
diff --git a/tests_interop/conftest.py b/tests_interop/conftest.py
index e75e519..8ae9769 100644
--- a/tests_interop/conftest.py
+++ b/tests_interop/conftest.py
@@ -76,7 +76,24 @@ def is_gossipsub():
 
 
 @pytest.fixture
-async def p2pds(num_p2pds, is_host_secure, is_gossipsub, unused_tcp_port_factory):
+def is_pubsub_signing():
+    return True
+
+
+@pytest.fixture
+def is_pubsub_signing_strict():
+    return True
+
+
+@pytest.fixture
+async def p2pds(
+    num_p2pds,
+    is_host_secure,
+    is_gossipsub,
+    unused_tcp_port_factory,
+    is_pubsub_signing,
+    is_pubsub_signing_strict,
+):
     p2pds: Union[Daemon, Exception] = await asyncio.gather(
         *[
             make_p2pd(
@@ -84,6 +101,8 @@ async def p2pds(num_p2pds, is_host_secure, is_gossipsub, unused_tcp_port_factory
                 unused_tcp_port_factory(),
                 is_host_secure,
                 is_gossipsub=is_gossipsub,
+                is_pubsub_signing=is_pubsub_signing,
+                is_pubsub_signing_strict=is_pubsub_signing_strict,
             )
             for _ in range(num_p2pds)
         ],
@@ -102,13 +121,14 @@ async def p2pds(num_p2pds, is_host_secure, is_gossipsub, unused_tcp_port_factory
 
 
 @pytest.fixture
-def pubsubs(num_hosts, hosts, is_gossipsub):
+def pubsubs(num_hosts, hosts, is_gossipsub, is_pubsub_signing_strict):
     if is_gossipsub:
         routers = GossipsubFactory.create_batch(num_hosts, **GOSSIPSUB_PARAMS._asdict())
     else:
         routers = FloodsubFactory.create_batch(num_hosts)
     _pubsubs = tuple(
-        PubsubFactory(host=host, router=router) for host, router in zip(hosts, routers)
+        PubsubFactory(host=host, router=router, strict_signing=is_pubsub_signing_strict)
+        for host, router in zip(hosts, routers)
     )
     yield _pubsubs
     # TODO: Clean up
diff --git a/tests_interop/test_pubsub.py b/tests_interop/test_pubsub.py
index 4e845d7..f67b47a 100644
--- a/tests_interop/test_pubsub.py
+++ b/tests_interop/test_pubsub.py
@@ -55,6 +55,9 @@ def validate_pubsub_msg(msg: rpc_pb2.Message, data: bytes, from_peer_id: ID) ->
     assert msg.data == data and msg.from_id == from_peer_id
 
 
+@pytest.mark.parametrize(
+    "is_pubsub_signing, is_pubsub_signing_strict", ((True, True), (False, False))
+)
 @pytest.mark.parametrize("is_gossipsub", (True, False))
 @pytest.mark.parametrize("num_hosts, num_p2pds", ((1, 2),))
 @pytest.mark.asyncio