diff --git a/com.unity.postprocessing/CHANGELOG.md b/com.unity.postprocessing/CHANGELOG.md
index 0b7b0388bd5..1084236fb0c 100644
--- a/com.unity.postprocessing/CHANGELOG.md
+++ b/com.unity.postprocessing/CHANGELOG.md
@@ -4,6 +4,16 @@ All notable changes to this package will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [4.0.0] - 2026-06-01
+
+### Deprecated
+- **The entire `com.unity.postprocessing` package is now deprecated and no longer actively developed.**
+  - For URP and HDRP projects, migrate to the integrated Volume-based post-processing system.
+  - For Built-in Render Pipeline projects, consider migrating to URP.
+  - See [Post-processing Overview](https://docs.unity3d.com/Manual/PostProcessingOverview.html) for migration guidance.
+- Added `[Obsolete]` attributes to `PostProcessVolume`, `PostProcessLayer`, and `PostProcessProfile` classes.
+- Added startup dialog notification that appears once per Unity session, with optional "Don't ask again on this computer" checkbox for permanent dismissal.
+
 ## [3.5.4] - 2026-03-13
 
 ### Fixed
diff --git a/com.unity.postprocessing/PostProcessing/Editor/DeprecationNotice.cs b/com.unity.postprocessing/PostProcessing/Editor/DeprecationNotice.cs
new file mode 100644
index 00000000000..dad88cee035
--- /dev/null
+++ b/com.unity.postprocessing/PostProcessing/Editor/DeprecationNotice.cs
@@ -0,0 +1,49 @@
+using UnityEditor;
+using UnityEngine;
+
+namespace UnityEngine.Rendering.PostProcessing
+{
+    [InitializeOnLoad]
+    internal static class DeprecationNotice
+    {
+        const string k_DialogOptOutKey = "PPv2.DeprecationNotice";
+        const string k_SessionShownKey = "PPv2.DeprecationNotice.SessionShown";
+
+        static DeprecationNotice()
+        {
+            // Check if user has permanently opted out
+            if (EditorPrefs.GetBool(k_DialogOptOutKey, false))
+                return;
+
+            // Check if already shown this session
+            if (SessionState.GetBool(k_SessionShownKey, false))
+                return;
+
+            // Mark as shown for this session
+            SessionState.SetBool(k_SessionShownKey, true);
+
+            // Use EditorApplication.update to show dialog after editor is fully initialized
+            EditorApplication.update += ShowDeprecationDialog;
+        }
+
+        static void ShowDeprecationDialog()
+        {
+            // Remove the callback so it only runs once
+            EditorApplication.update -= ShowDeprecationDialog;
+
+            // Use Unity's built-in opt-out dialog with ForThisUser (permanent checkbox)
+            // Clicking OK without checkbox = dismiss for this session only
+            // Checking the checkbox = dismiss permanently
+            EditorUtility.DisplayDialog(
+                "Post Processing Stack v2 - Deprecated",
+                "This package is deprecated and no longer actively developed.\n\n" +
+                "For URP and HDRP projects, use the integrated post-processing via the Volume framework.\n\n" +
+                "For Built-in Render Pipeline projects, consider migrating to URP.\n\n" +
+                "See documentation for migration guidance:\n" +
+                "https://docs.unity3d.com/Manual/PostProcessingOverview.html",
+                "OK",
+                DialogOptOutDecisionType.ForThisUser,
+                k_DialogOptOutKey);
+        }
+    }
+}
diff --git a/com.unity.postprocessing/PostProcessing/Editor/DeprecationNotice.cs.meta b/com.unity.postprocessing/PostProcessing/Editor/DeprecationNotice.cs.meta
new file mode 100644
index 00000000000..13b59182ef5
--- /dev/null
+++ b/com.unity.postprocessing/PostProcessing/Editor/DeprecationNotice.cs.meta
@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6
\ No newline at end of file
diff --git a/com.unity.postprocessing/PostProcessing/Editor/EffectListEditor.cs b/com.unity.postprocessing/PostProcessing/Editor/EffectListEditor.cs
index 52f35021056..676483c9791 100644
--- a/com.unity.postprocessing/PostProcessing/Editor/EffectListEditor.cs
+++ b/com.unity.postprocessing/PostProcessing/Editor/EffectListEditor.cs
@@ -5,6 +5,8 @@
 using UnityEngine.Assertions;
 using UnityEngine.Rendering.PostProcessing;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEditor.Rendering.PostProcessing
 {
     /// <summary>
@@ -343,3 +345,5 @@ PostProcessEffectSettings CreateNewEffect(Type type)
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Editor/PostProcessDebugEditor.cs b/com.unity.postprocessing/PostProcessing/Editor/PostProcessDebugEditor.cs
index 2455861e6fa..16224f756a0 100644
--- a/com.unity.postprocessing/PostProcessing/Editor/PostProcessDebugEditor.cs
+++ b/com.unity.postprocessing/PostProcessing/Editor/PostProcessDebugEditor.cs
@@ -1,6 +1,8 @@
 using UnityEngine;
 using UnityEngine.Rendering.PostProcessing;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEditor.Rendering.PostProcessing
 {
     [CustomEditor(typeof(PostProcessDebug))]
@@ -142,3 +144,5 @@ void DoOverlayGUI(DebugOverlay overlay, params SerializedProperty[] settings)
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Editor/PostProcessLayerEditor.cs b/com.unity.postprocessing/PostProcessing/Editor/PostProcessLayerEditor.cs
index 1c12846090c..7ee7c6b3710 100644
--- a/com.unity.postprocessing/PostProcessing/Editor/PostProcessLayerEditor.cs
+++ b/com.unity.postprocessing/PostProcessing/Editor/PostProcessLayerEditor.cs
@@ -6,6 +6,8 @@
 using UnityEditorInternal;
 using System.IO;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEditor.Rendering.PostProcessing
 {
     using SerializedBundleRef = PostProcessLayer.SerializedBundleRef;
@@ -427,3 +429,5 @@ void ExportFrameToExr(ExportMode mode)
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Editor/PostProcessProfileEditor.cs b/com.unity.postprocessing/PostProcessing/Editor/PostProcessProfileEditor.cs
index 2f586c3375e..d7a49c8db82 100644
--- a/com.unity.postprocessing/PostProcessing/Editor/PostProcessProfileEditor.cs
+++ b/com.unity.postprocessing/PostProcessing/Editor/PostProcessProfileEditor.cs
@@ -1,5 +1,7 @@
 using UnityEngine.Rendering.PostProcessing;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEditor.Rendering.PostProcessing
 {
     [CustomEditor(typeof(PostProcessProfile))]
@@ -27,3 +29,5 @@ public override void OnInspectorGUI()
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Editor/PostProcessVolumeEditor.cs b/com.unity.postprocessing/PostProcessing/Editor/PostProcessVolumeEditor.cs
index a8de4405b51..53709570a6d 100644
--- a/com.unity.postprocessing/PostProcessing/Editor/PostProcessVolumeEditor.cs
+++ b/com.unity.postprocessing/PostProcessing/Editor/PostProcessVolumeEditor.cs
@@ -1,6 +1,8 @@
 using UnityEngine;
 using UnityEngine.Rendering.PostProcessing;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEditor.Rendering.PostProcessing
 {
     [CanEditMultipleObjects, CustomEditor(typeof(PostProcessVolume))]
@@ -164,3 +166,5 @@ public PostProcessProfile profileRef
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Editor/Tools/ProfileFactory.cs b/com.unity.postprocessing/PostProcessing/Editor/Tools/ProfileFactory.cs
index 75040d96580..46e67404501 100644
--- a/com.unity.postprocessing/PostProcessing/Editor/Tools/ProfileFactory.cs
+++ b/com.unity.postprocessing/PostProcessing/Editor/Tools/ProfileFactory.cs
@@ -4,6 +4,8 @@
 using UnityEngine.SceneManagement;
 using UnityEngine.Rendering.PostProcessing;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEditor.Rendering.PostProcessing
 {
     /// <summary>
@@ -96,3 +98,5 @@ public override void Action(int instanceId, string pathName, string resourceFile
     }
 #endif
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Editor/Tools/VolumeFactory.cs b/com.unity.postprocessing/PostProcessing/Editor/Tools/VolumeFactory.cs
index c518a079bc9..03d5021fdc6 100644
--- a/com.unity.postprocessing/PostProcessing/Editor/Tools/VolumeFactory.cs
+++ b/com.unity.postprocessing/PostProcessing/Editor/Tools/VolumeFactory.cs
@@ -1,6 +1,8 @@
 using UnityEngine;
 using UnityEngine.Rendering.PostProcessing;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEditor.Rendering.PostProcessing
 {
     internal static class VolumeFactory
@@ -19,3 +21,5 @@ static void CreateVolume()
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessDebug.cs b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessDebug.cs
index c63f333086a..e9e80a77c6a 100644
--- a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessDebug.cs
+++ b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessDebug.cs
@@ -1,3 +1,5 @@
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEngine.Rendering.PostProcessing
 {
     /// <summary>
@@ -158,3 +160,5 @@ void DrawMonitor(ref Rect rect, Monitor monitor, bool enabled)
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessEffectRenderer.cs b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessEffectRenderer.cs
index 9a5b293bea8..2ae39fdaef3 100644
--- a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessEffectRenderer.cs
+++ b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessEffectRenderer.cs
@@ -1,3 +1,5 @@
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEngine.Rendering.PostProcessing
 {
     /// <summary>
@@ -76,3 +78,5 @@ internal override void SetSettings(PostProcessEffectSettings settings)
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessLayer.cs b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessLayer.cs
index b2ed5fd63c8..edd9cbce66d 100644
--- a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessLayer.cs
+++ b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessLayer.cs
@@ -21,6 +21,7 @@ namespace UnityEngine.Rendering.PostProcessing
     [DisallowMultipleComponent, ImageEffectAllowedInSceneView]
     [AddComponentMenu("Rendering/Post-process Layer", 1000)]
     [RequireComponent(typeof(Camera))]
+    [System.Obsolete("Post Processing Stack v2 is deprecated. Use URP or HDRP's Volume-based post-processing instead.", false)]
     public sealed class PostProcessLayer : MonoBehaviour
     {
         /// <summary>
diff --git a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessManager.cs b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessManager.cs
index 0a942fd00d3..80ce842afe6 100644
--- a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessManager.cs
+++ b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessManager.cs
@@ -3,6 +3,8 @@
 using System.Linq;
 using UnityEngine.Assertions;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEngine.Rendering.PostProcessing
 {
     /// <summary>
@@ -460,3 +462,5 @@ static bool IsVolumeRenderedByCamera(PostProcessVolume volume, Camera camera)
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessProfile.cs b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessProfile.cs
index 7f4b5430e4d..97447d57dcd 100644
--- a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessProfile.cs
+++ b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessProfile.cs
@@ -7,6 +7,7 @@ namespace UnityEngine.Rendering.PostProcessing
     /// An asset holding a set of post-processing settings to use with a <see cref="PostProcessVolume"/>.
     /// </summary>
     /// <seealso cref="PostProcessVolume"/>
+    [System.Obsolete("Post Processing Stack v2 is deprecated. Use URP or HDRP's Volume-based post-processing instead.", false)]
     public sealed class PostProcessProfile : ScriptableObject
     {
         /// <summary>
diff --git a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessRenderContext.cs b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessRenderContext.cs
index 4ca66cace71..585517d1f70 100644
--- a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessRenderContext.cs
+++ b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessRenderContext.cs
@@ -1,5 +1,7 @@
 using System.Collections.Generic;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEngine.Rendering.PostProcessing
 {
 #if (ENABLE_VR_MODULE && ENABLE_VR)
@@ -434,3 +436,5 @@ public void UpdateSinglePassStereoState(bool isTAAEnabled, bool isAOEnabled, boo
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessVolume.cs b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessVolume.cs
index fa787750e39..7e21a48b90b 100644
--- a/com.unity.postprocessing/PostProcessing/Runtime/PostProcessVolume.cs
+++ b/com.unity.postprocessing/PostProcessing/Runtime/PostProcessVolume.cs
@@ -61,6 +61,7 @@ namespace UnityEngine.Rendering.PostProcessing
     [ExecuteInEditMode]
 #endif
     [AddComponentMenu("Rendering/Post-process Volume", 1001)]
+    [System.Obsolete("Post Processing Stack v2 is deprecated. Use URP or HDRP's Volume-based post-processing instead.", false)]
     public sealed class PostProcessVolume : MonoBehaviour
     {
         /// <summary>
diff --git a/com.unity.postprocessing/PostProcessing/Runtime/Utils/RuntimeUtilities.cs b/com.unity.postprocessing/PostProcessing/Runtime/Utils/RuntimeUtilities.cs
index cf0eb3588b0..14e44977dc2 100644
--- a/com.unity.postprocessing/PostProcessing/Runtime/Utils/RuntimeUtilities.cs
+++ b/com.unity.postprocessing/PostProcessing/Runtime/Utils/RuntimeUtilities.cs
@@ -14,6 +14,8 @@
 using UnityEngine.Assemblies;
 #endif
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace UnityEngine.Rendering.PostProcessing
 {
     using SceneManagement;
@@ -1330,3 +1332,5 @@ public static string GetFieldPath<TType, TValue>(Expression<Func<TType, TValue>>
         #endregion
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete
diff --git a/com.unity.postprocessing/package.json b/com.unity.postprocessing/package.json
index f5d7e607d4b..9689f8b9684 100644
--- a/com.unity.postprocessing/package.json
+++ b/com.unity.postprocessing/package.json
@@ -1,10 +1,11 @@
 {
     "name": "com.unity.postprocessing",
-    "version": "3.5.4",
+    "version": "4.0.0",
     "displayName": "Post Processing",
     "unity": "2019.4",
     "unityRelease": "19f1",
-    "description": "The post-processing stack (v2) comes with a collection of effects and image filters you can apply to your cameras to improve the visuals of your games.",
+    "deprecated": "Post Processing Stack v2 is deprecated and no longer actively developed. For URP and HDRP projects, use the integrated post-processing via the Volume framework. For Built-in Render Pipeline projects, consider migrating to URP. See https://docs.unity3d.com/Manual/PostProcessingOverview.html for migration guidance.",
+    "description": "[DEPRECATED] The post-processing stack (v2) comes with a collection of effects and image filters you can apply to your cameras to improve the visuals of your games. This package is deprecated - use URP/HDRP Volume-based post-processing instead.",
   "dependencies": {
     "com.unity.modules.physics": "1.0.0",
     "com.unity.modules.imgui": "1.0.0"