diff --git a/src/GameLogic/GameContext.cs b/src/GameLogic/GameContext.cs
index 8d9d42e74..0da19768a 100644
--- a/src/GameLogic/GameContext.cs
+++ b/src/GameLogic/GameContext.cs
@@ -122,6 +122,11 @@ public GameContext(GameConfiguration configuration, IPersistenceContextProvider
     /// </summary>
     public IDictionary<string, Player> PlayersByCharacterName { get; } = new ConcurrentDictionary<string, Player>();
 
+    /// <summary>
+    /// Gets the state of the active self defenses.
+    /// </summary>
+    public ConcurrentDictionary<(Player Attacker, Player Defender), DateTime> SelfDefenseState { get; } = new();
+
     /// <inheritdoc />
     public ILoggerFactory LoggerFactory { get; }
 
diff --git a/src/GameLogic/IGameContext.cs b/src/GameLogic/IGameContext.cs
index 2407b5e0a..62b839b5f 100644
--- a/src/GameLogic/IGameContext.cs
+++ b/src/GameLogic/IGameContext.cs
@@ -9,6 +9,7 @@ namespace MUnique.OpenMU.GameLogic;
 using MUnique.OpenMU.Pathfinding;
 using MUnique.OpenMU.Persistence;
 using MUnique.OpenMU.PlugIns;
+using System.Collections.Concurrent;
 
 /// <summary>
 /// The context of the game.
@@ -75,6 +76,11 @@ public interface IGameContext
     /// </summary>
     IObjectPool<PathFinder> PathFinderPool { get; }
 
+    /// <summary>
+    /// Gets the state of the active self defenses.
+    /// </summary>
+    ConcurrentDictionary<(Player Attacker, Player Defender), DateTime> SelfDefenseState { get; }
+
     /// <summary>
     /// Gets the initialized maps which are hosted on this context.
     /// </summary>
diff --git a/src/GameLogic/Player.cs b/src/GameLogic/Player.cs
index 5180b22ef..f59d7e96f 100644
--- a/src/GameLogic/Player.cs
+++ b/src/GameLogic/Player.cs
@@ -534,6 +534,23 @@ public async ValueTask KillInstantlyAsync()
         await this.OnDeathAsync(null).ConfigureAwait(false);
     }
 
+    /// <summary>
+    /// Determines whether the self defense is active for the specified attacker.
+    /// </summary>
+    /// <param name="attacker">The attacker.</param>
+    /// <returns>
+    ///   <c>true</c> if the self defense is active for the specified attacker; otherwise, <c>false</c>.
+    /// </returns>
+    public bool IsSelfDefenseActive(Player attacker)
+    {
+        if (this.GameContext.SelfDefenseState.TryGetValue((attacker, this), out var timeout))
+        {
+            return timeout > DateTime.UtcNow;
+        }
+
+        return false;
+    }
+
     /// <inheritdoc/>
     public async ValueTask AttackByAsync(IAttacker attacker, SkillEntry? skill, bool isCombo)
     {
@@ -1636,7 +1653,7 @@ private async ValueTask OnDeathAsync(IAttacker? killer)
             && !(killerAfterKilled.GuildWarContext?.Score is { } score && score == this.GuildWarContext?.Score)
             && this.CurrentMiniGame?.AllowPlayerKilling is not true)
         {
-            await killerAfterKilled.AfterKilledPlayerAsync().ConfigureAwait(false);
+            await killerAfterKilled.AfterKilledPlayerAsync(this).ConfigureAwait(false);
         }
 
         // TODO: Drop items
@@ -1679,9 +1696,27 @@ async Task RespawnAsync(CancellationToken cancellationToken)
     /// Is called after the player killed a <see cref="Player"/>.
     /// Increment PK Level.
     /// </summary>
-    private async ValueTask AfterKilledPlayerAsync()
+    private async ValueTask AfterKilledPlayerAsync(Player killedPlayer)
     {
-        // TODO: Self Defense System
+        var killedPlayerState = killedPlayer.SelectedCharacter?.State;
+        if (killedPlayerState is null)
+        {
+            return;
+        }
+
+        if (killedPlayerState >= HeroState.PlayerKiller1stStage)
+        {
+            // Killing PKs is allowed.
+            return;
+        }
+
+        if (killedPlayerState <= HeroState.PlayerKillWarning
+            && this.IsSelfDefenseActive(killedPlayer))
+        {
+            // Self defense is allowed.
+            return;
+        }
+
         if (this._selectedCharacter!.State != HeroState.PlayerKiller2ndStage)
         {
             if (this._selectedCharacter.State < HeroState.Normal)
diff --git a/src/GameLogic/PlugIns/SelfDefensePlugIn.cs b/src/GameLogic/PlugIns/SelfDefensePlugIn.cs
new file mode 100644
index 000000000..347b93621
--- /dev/null
+++ b/src/GameLogic/PlugIns/SelfDefensePlugIn.cs
@@ -0,0 +1,86 @@
+// <copyright file="SelfDefensePlugIn.cs" company="MUnique">
+// Licensed under the MIT License. See LICENSE file in the project root for full license information.
+// </copyright>
+
+using MUnique.OpenMU.GameLogic.NPC;
+
+namespace MUnique.OpenMU.GameLogic.PlugIns;
+
+using System;
+using System.Runtime.InteropServices;
+using MUnique.OpenMU.GameLogic.Views;
+using MUnique.OpenMU.Interfaces;
+using MUnique.OpenMU.PlugIns;
+
+/// <summary>
+/// Updates the state of the active self defenses on every second and every hit.
+/// </summary>
+[PlugIn(nameof(SelfDefensePlugIn), "Updates the state of the self defense system.")]
+[Guid("3E702A15-653A-48EF-899C-4CDB2239A90C")]
+public class SelfDefensePlugIn : IPeriodicTaskPlugIn, IAttackableGotHitPlugIn
+{
+    private readonly TimeSpan _selfDefenseTime = TimeSpan.FromSeconds(60);
+
+    /// <inheritdoc />
+    public async ValueTask ExecuteTaskAsync(GameContext gameContext)
+    {
+        var timedOut = gameContext.SelfDefenseState.Where(s => DateTime.UtcNow.Subtract(s.Value) >= _selfDefenseTime).ToList();
+        foreach (var (pair, lastAttack) in timedOut)
+        {
+            if (gameContext.SelfDefenseState.Remove(pair, out _))
+            {
+                await this.EndSelfDefenseAsync(pair.Attacker, pair.Defender).ConfigureAwait(false);
+            }
+        }
+    }
+
+    /// <inheritdoc />
+    public void AttackableGotHit(IAttackable attackable, IAttacker attacker, HitInfo hitInfo)
+    {
+        var defender = attackable as Player ?? (attackable as Monster)?.SummonedBy;
+        var attackerPlayer = attacker as Player ?? (attackable as Monster)?.SummonedBy;
+        if (defender is null || attackerPlayer is null)
+        {
+            return;
+        }
+
+        if (defender.SelectedCharacter?.State >= HeroState.PlayerKiller1stStage)
+        {
+            // PKs have no right to self defense.
+            return;
+        }
+
+        if (attackerPlayer.IsSelfDefenseActive(defender))
+        {
+            // Attacking during self defense period does not initiate another self defense.
+            return;
+        }
+
+        if (hitInfo is { HealthDamage: 0, ShieldDamage: 0 } || hitInfo.Attributes.HasFlag(DamageAttributes.Reflected))
+        {
+            return;
+        }
+
+        var now = DateTime.UtcNow;
+        var gameContext = defender.GameContext;
+        gameContext.SelfDefenseState.AddOrUpdate((attackerPlayer, defender), tuple =>
+        {
+            _ = this.BeginSelfDefenseAsync(attackerPlayer, defender);
+            return now;
+        }, (tuple, time) => now);
+    }
+
+    private async ValueTask BeginSelfDefenseAsync(Player attacker, Player defender)
+    {
+        var message = $"Self defense is initiated by {attacker.Name}'s attack to {defender.Name}!";
+        await defender.InvokeViewPlugInAsync<IShowMessagePlugIn>(p => p.ShowMessageAsync(message, MessageType.BlueNormal));
+        await attacker.InvokeViewPlugInAsync<IShowMessagePlugIn>(p => p.ShowMessageAsync(message, MessageType.BlueNormal));
+    }
+
+    private async ValueTask EndSelfDefenseAsync(Player attacker, Player defender)
+    {
+        var message = $"Self defense of {defender.Name} against {attacker.Name} diminishes.";
+        await defender.InvokeViewPlugInAsync<IShowMessagePlugIn>(p => p.ShowMessageAsync(message, MessageType.BlueNormal)).ConfigureAwait(false);
+        await attacker.InvokeViewPlugInAsync<IShowMessagePlugIn>(p => p.ShowMessageAsync(message, MessageType.BlueNormal)).ConfigureAwait(false);
+    }
+}
\ No newline at end of file