diff --git a/paircd/handler/nick.py b/paircd/handler/nick.py
index f0b319f..46620f6 100644
--- a/paircd/handler/nick.py
+++ b/paircd/handler/nick.py
@@ -1,6 +1,7 @@
 from paircd.client import Client
 from paircd.command_handler import CommandHandler
 from paircd.message import Message
+from paircd.reply import ERR_NICKNAMEINUSE
 from paircd.server import Server
 
 
@@ -11,6 +12,13 @@ class NickHandler(CommandHandler):
     async def handle(self, server: Server, client: Client, msg: Message) -> None:
         nickname = msg.args[0]
 
+        existing_client = server.get_client_by_name(nickname)
+        if existing_client is not None:
+            if client != existing_client:
+                target = client.nickname if client.nickname else nickname
+                client.write_message(ERR_NICKNAMEINUSE(target, nickname))
+            return
+
         # Remove stale references if they exist
         if client.nickname:
             server.remove_client_by_name(client.nickname)
diff --git a/paircd/reply.py b/paircd/reply.py
index 461af08..07050a2 100644
--- a/paircd/reply.py
+++ b/paircd/reply.py
@@ -35,6 +35,7 @@ ERR_NOSUCHSERVER = reply_fn(402, "{0} :No such server")
 ERR_NOSUCHCHANNEL = reply_fn(403, "{0} :No such channel")
 ERR_CANNOTSENDTOCHAN = reply_fn(404, "{0} :Cannot send to channel")
 ERR_TOOMANYCHANNELS = reply_fn(405, "{0} :You have joined too many channels")
+ERR_NICKNAMEINUSE = reply_fn(433, "{0} :Nickname is already in use")
 ERR_NOTEXTTOSEND = reply_fn(412, ":No text to send")
 ERR_NOTREGISTERED = reply_fn(451, ":You have not registered")
 
diff --git a/paircd/server.py b/paircd/server.py
index 394be59..db491d1 100644
--- a/paircd/server.py
+++ b/paircd/server.py
@@ -1,6 +1,6 @@
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import Dict
+from typing import Dict, Optional
 
 from paircd.client import Client
 from paircd.channel import Channel
@@ -20,5 +20,8 @@ class Server:
             self.channels_by_name[name] = Channel(name=name)
         return self.channels_by_name[name]
 
+    def get_client_by_name(self, name: str) -> Optional[Client]:
+        return self.clients_by_nick.get(name)
+
     def remove_client_by_name(self, name: str) -> None:
         del self.clients_by_nick[name]