diff --git a/app/models/user.rb b/app/models/user.rb
index 20ee1c40c..6503881a3 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -14,8 +14,6 @@ class User < ApplicationRecord
   has_many :pinned_cards, through: :pins, source: :card
   has_many :exports, class_name: "Account::Export", dependent: :destroy
 
-  scope :with_avatars, -> { preload(:account, :avatar_attachment) }
-
   def deactivate
     transaction do
       accesses.destroy_all
diff --git a/app/models/user/avatar.rb b/app/models/user/avatar.rb
index c635edddb..400738cb1 100644
--- a/app/models/user/avatar.rb
+++ b/app/models/user/avatar.rb
@@ -2,13 +2,20 @@ module User::Avatar
   extend ActiveSupport::Concern
 
   ALLOWED_AVATAR_CONTENT_TYPES = %w[ image/jpeg image/png image/gif image/webp ].freeze
+  MAX_AVATAR_DIMENSIONS = { width: 4096, height: 4096 }.freeze
 
   included do
     has_one_attached :avatar do |attachable|
       attachable.variant :thumb, resize_to_fill: [ 256, 256 ], process: :immediately
     end
 
-    validate :avatar_content_type_allowed
+    scope :with_avatars, -> { preload(:account, :avatar_attachment) }
+
+    validate :avatar_content_type_allowed, :avatar_dimensions_allowed, if: :avatar_attached?
+  end
+
+  def avatar_attached?
+    avatar.attached?
   end
 
   def avatar_thumbnail
@@ -17,8 +24,23 @@ module User::Avatar
 
   private
     def avatar_content_type_allowed
-      if avatar.attached? && !ALLOWED_AVATAR_CONTENT_TYPES.include?(avatar.content_type)
+      if !ALLOWED_AVATAR_CONTENT_TYPES.include?(avatar.content_type)
         errors.add(:avatar, "must be a JPEG, PNG, GIF, or WebP image")
       end
     end
+
+    def avatar_dimensions_allowed
+      return unless avatar.blob.analyzed? || avatar.blob.analyze
+
+      width = avatar.blob.metadata[:width]
+      height = avatar.blob.metadata[:height]
+
+      if width && width > MAX_AVATAR_DIMENSIONS[:width]
+        errors.add(:avatar, "width must be less than #{MAX_AVATAR_DIMENSIONS[:width]}px")
+      end
+
+      if height && height > MAX_AVATAR_DIMENSIONS[:height]
+        errors.add(:avatar, "height must be less than #{MAX_AVATAR_DIMENSIONS[:height]}px")
+      end
+    end
 end
diff --git a/test/controllers/users_controller_test.rb b/test/controllers/users_controller_test.rb
index 20008dcee..8e99e4cef 100644
--- a/test/controllers/users_controller_test.rb
+++ b/test/controllers/users_controller_test.rb
@@ -65,7 +65,7 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
     assert_response :forbidden
   end
 
-  test "update with invalid avatar shows validation error" do
+  test "update with invalid avatar content type shows validation error" do
     sign_in_as :kevin
 
     svg_file = fixture_file_upload("avatar.svg", "image/svg+xml")
@@ -76,6 +76,18 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
     assert_select ".txt-negative", text: /must be a JPEG, PNG, GIF, or WebP image/
   end
 
+  test "update with oversized avatar shows validation error" do
+    sign_in_as :kevin
+
+    png_file = fixture_file_upload("avatar.png", "image/png")
+
+    ActiveStorage::Analyzer::ImageAnalyzer::Vips.any_instance.stubs(:metadata).returns({ width: 5000, height: 100 })
+
+    put user_path(users(:kevin)), params: { user: { avatar: png_file } }
+    assert_response :unprocessable_entity
+    assert_select ".txt-negative", text: /width must be less than 4096px/
+  end
+
   test "update with valid avatar" do
     sign_in_as :kevin
 
diff --git a/test/models/user/avatar_test.rb b/test/models/user/avatar_test.rb
index ed502d7ab..fcaf05d0b 100644
--- a/test/models/user/avatar_test.rb
+++ b/test/models/user/avatar_test.rb
@@ -33,4 +33,27 @@ class User::AvatarTest < ActiveSupport::TestCase
 
     assert users(:david).avatar.variant(:thumb).processed?
   end
+
+  test "rejects images that are too wide" do
+    users(:david).avatar.attach(io: File.open(file_fixture("avatar.png")), filename: "avatar.png", content_type: "image/png")
+    users(:david).avatar.blob.update!(metadata: { analyzed: true, width: 5000, height: 100 })
+
+    assert_not users(:david).valid?
+    assert_includes users(:david).errors[:avatar], "width must be less than #{User::Avatar::MAX_AVATAR_DIMENSIONS[:width]}px"
+  end
+
+  test "rejects images that are too tall" do
+    users(:david).avatar.attach(io: File.open(file_fixture("avatar.png")), filename: "avatar.png", content_type: "image/png")
+    users(:david).avatar.blob.update!(metadata: { analyzed: true, width: 100, height: 5000 })
+
+    assert_not users(:david).valid?
+    assert_includes users(:david).errors[:avatar], "height must be less than #{User::Avatar::MAX_AVATAR_DIMENSIONS[:height]}px"
+  end
+
+  test "accepts images within dimension limits" do
+    users(:david).avatar.attach(io: File.open(file_fixture("avatar.png")), filename: "avatar.png", content_type: "image/png")
+    users(:david).avatar.blob.update!(metadata: { analyzed: true, width: 4096, height: 4096 })
+
+    assert users(:david).valid?
+  end
 end