diff --git a/app/controllers/forum_posts_controller.rb b/app/controllers/forum_posts_controller.rb
index e6ef8ea68..0ab945b58 100644
--- a/app/controllers/forum_posts_controller.rb
+++ b/app/controllers/forum_posts_controller.rb
@@ -55,12 +55,16 @@ class ForumPostsController < ApplicationController
   def destroy
     @forum_post = authorize ForumPost.find(params[:id])
     @forum_post.delete!
+
+    flash[:notice] = @forum_post.errors.none? ? "Post deleted" : @forum_post.errors.full_messages.join("; ")
     respond_with(@forum_post)
   end
 
   def undelete
     @forum_post = authorize ForumPost.find(params[:id])
     @forum_post.undelete!
+
+    flash[:notice] = @forum_post.errors.none? ? "Post undeleted" : @forum_post.errors.full_messages.join("; ")
     respond_with(@forum_post)
   end
 end
diff --git a/app/models/forum_post.rb b/app/models/forum_post.rb
index 5bd66a95b..17ea1d6f9 100644
--- a/app/models/forum_post.rb
+++ b/app/models/forum_post.rb
@@ -15,13 +15,14 @@ class ForumPost < ApplicationRecord
   has_one :bulk_update_request
 
   validates :body, presence: true, length: { maximum: 200_000 }, if: :body_changed?
+  validate :validate_deletion_of_original_post
+  validate :validate_undeletion_of_post
 
   before_create :autoreport_spam
   before_save :handle_reports_on_deletion
   after_create :update_topic_updated_at_on_create
   after_update :update_topic_updated_at_on_update_for_original_posts
   after_destroy :update_topic_updated_at_on_destroy
-  after_save :delete_topic_if_original_post
   after_update(:if => ->(rec) {rec.updater_id != rec.creator_id}) do |rec|
     ModAction.log("#{CurrentUser.user.name} updated forum ##{rec.id}", :forum_post_update)
   end
@@ -83,6 +84,18 @@ class ForumPost < ApplicationRecord
     votes.exists?(creator_id: user.id, score: score)
   end
 
+  def validate_deletion_of_original_post
+    if is_original_post? && is_deleted? && !topic.is_deleted?
+      errors.add(:base, "Can't delete original post without deleting the topic first")
+    end
+  end
+
+  def validate_undeletion_of_post
+    if topic.is_deleted? && !is_deleted?
+      errors.add(:base, "Can't undelete post without undeleting the topic first")
+    end
+  end
+
   def autoreport_spam
     if SpamDetector.new(self, user_ip: CurrentUser.ip_addr).spam?
       moderation_reports << ModerationReport.new(creator: User.system, reason: "Spam.")
@@ -153,12 +166,6 @@ class ForumPost < ApplicationRecord
     end
   end
 
-  def delete_topic_if_original_post
-    if is_deleted? && is_original_post?
-      topic.update_attribute(:is_deleted, true)
-    end
-  end
-
   def handle_reports_on_deletion
     return unless moderation_reports.pending.present? && is_deleted_change == [false, true]
 
diff --git a/app/models/forum_topic.rb b/app/models/forum_topic.rb
index 76056a957..bdf6cea95 100644
--- a/app/models/forum_topic.rb
+++ b/app/models/forum_topic.rb
@@ -24,12 +24,13 @@ class ForumTopic < ApplicationRecord
   has_many :tag_implications
 
   validates :title, presence: true, length: { maximum: 200 }, if: :title_changed?
-  validates_associated :original_post
   validates :category_id, inclusion: { in: CATEGORIES.keys }
   validates :min_level, inclusion: { in: MIN_LEVELS.values }
 
   accepts_nested_attributes_for :original_post
-  after_update :update_orignal_post
+
+  after_update :update_posts_on_deletion_or_undeletion
+  after_update :update_original_post
   after_save(:if => ->(rec) {rec.is_locked? && rec.saved_change_to_is_locked?}) do |rec|
     ModAction.log("locked forum topic ##{id} (title: #{title})", :forum_topic_lock)
   end
@@ -179,7 +180,14 @@ class ForumTopic < ApplicationRecord
     (response_count / Danbooru.config.posts_per_page.to_f).ceil
   end
 
-  def update_orignal_post
+  # Delete all posts when the topic is deleted. Undelete all posts when the topic is undeleted.
+  def update_posts_on_deletion_or_undeletion
+    if saved_change_to_is_deleted?
+      forum_posts.update!(is_deleted: is_deleted) # XXX depends on current user
+    end
+  end
+
+  def update_original_post
     original_post&.update_columns(:updater_id => updater.id, :updated_at => Time.now)
   end
 
diff --git a/app/policies/forum_post_policy.rb b/app/policies/forum_post_policy.rb
index bbcc370c1..641b3e6e8 100644
--- a/app/policies/forum_post_policy.rb
+++ b/app/policies/forum_post_policy.rb
@@ -18,11 +18,11 @@ class ForumPostPolicy < ApplicationPolicy
   end
 
   def destroy?
-    unbanned? && show? && user.is_moderator?
+    unbanned? && show? && !record.is_deleted? && user.is_moderator?
   end
 
   def undelete?
-    unbanned? && show? && user.is_moderator?
+    unbanned? && show? && record.is_deleted? && !record.topic.is_deleted? && user.is_moderator?
   end
 
   def reply?
diff --git a/app/views/forum_posts/destroy.js.erb b/app/views/forum_posts/destroy.js.erb
index 7ccf4d7cd..345366b9b 100644
--- a/app/views/forum_posts/destroy.js.erb
+++ b/app/views/forum_posts/destroy.js.erb
@@ -1 +1 @@
-$("#forum_post_<%= @forum_post.id %>").replaceWith("<%= j render(ForumPostComponent.new(forum_post: @forum_post, current_user: CurrentUser.user)) %>");
+location.reload();
diff --git a/app/views/forum_posts/undelete.js.erb b/app/views/forum_posts/undelete.js.erb
index 7ccf4d7cd..345366b9b 100644
--- a/app/views/forum_posts/undelete.js.erb
+++ b/app/views/forum_posts/undelete.js.erb
@@ -1 +1 @@
-$("#forum_post_<%= @forum_post.id %>").replaceWith("<%= j render(ForumPostComponent.new(forum_post: @forum_post, current_user: CurrentUser.user)) %>");
+location.reload();
diff --git a/script/fixes/095_delete_forum_posts_in_deleted_topics.rb b/script/fixes/095_delete_forum_posts_in_deleted_topics.rb
new file mode 100755
index 000000000..01629a732
--- /dev/null
+++ b/script/fixes/095_delete_forum_posts_in_deleted_topics.rb
@@ -0,0 +1,17 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+with_confirmation do
+  CurrentUser.scoped(User.system) do
+    # delete all posts in deleted topics
+    ForumPost.undeleted.where(topic: ForumTopic.deleted).update_all(is_deleted: true, updated_at: Time.zone.now, updater_id: User.system.id)
+
+    # undelete all deleted OPs in active topics
+    ForumPost.deleted.where(topic: ForumTopic.undeleted).find_each do |forum_post|
+      if forum_post.is_original_post?
+        forum_post.update_columns(is_deleted: false, updated_at: Time.zone.now, updater_id: User.system.id)
+      end
+    end
+  end
+end
diff --git a/test/functional/forum_posts_controller_test.rb b/test/functional/forum_posts_controller_test.rb
index aa2e284f9..dfacd294e 100644
--- a/test/functional/forum_posts_controller_test.rb
+++ b/test/functional/forum_posts_controller_test.rb
@@ -219,24 +219,36 @@ class ForumPostsControllerTest < ActionDispatch::IntegrationTest
 
     context "destroy action" do
       should "allow mods to delete posts" do
-        delete_auth forum_post_path(@forum_post), @mod
-        assert_redirected_to(forum_post_path(@forum_post))
-        assert_equal(true, @forum_post.reload.is_deleted?)
+        @forum_reply = as(@user) { create(:forum_post, topic: @forum_topic, creator: @user) }
+        delete_auth forum_post_path(@forum_reply), @mod
+
+        assert_redirected_to(@forum_reply)
+        assert_equal(true, @forum_reply.reload.is_deleted?)
       end
 
       should "not allow users to delete their own posts" do
-        delete_auth forum_post_path(@forum_post), @user
+        @forum_reply = as(@user) { create(:forum_post, topic: @forum_topic, creator: @user) }
+        delete_auth forum_post_path(@forum_reply), @user
+
         assert_response 403
-        assert_equal(false, @forum_post.reload.is_deleted?)
+        assert_equal(false, @forum_reply.reload.is_deleted?)
+      end
+
+      should "not allow deleting the OP of an active topic" do
+        delete_auth forum_post_path(@forum_topic.original_post), @mod
+
+        assert_redirected_to(@forum_topic.original_post)
+        assert_equal(false, @forum_topic.original_post.is_deleted?)
       end
 
       should "mark all pending moderation reports against the post as handled" do
-        report1 = create(:moderation_report, model: @forum_post, status: :pending)
-        report2 = create(:moderation_report, model: @forum_post, status: :rejected)
-        delete_auth forum_post_path(@forum_post), @mod
+        forum_reply = as(@user) { create(:forum_post, topic: @forum_topic, creator: @user) }
+        report1 = create(:moderation_report, model: forum_reply, status: :pending)
+        report2 = create(:moderation_report, model: forum_reply, status: :rejected)
+        delete_auth forum_post_path(forum_reply), @mod
 
-        assert_redirected_to(forum_post_path(@forum_post))
-        assert_equal(true, @forum_post.reload.is_deleted?)
+        assert_redirected_to(forum_post_path(forum_reply))
+        assert_equal(true, forum_reply.reload.is_deleted?)
         assert_equal(true, report1.reload.handled?)
         assert_equal(true, report2.reload.rejected?)
         assert_equal(1, ModAction.moderation_report_handled.where(creator: @mod).count)
@@ -245,17 +257,30 @@ class ForumPostsControllerTest < ActionDispatch::IntegrationTest
 
     context "undelete action" do
       should "allow mods to undelete posts" do
-        as(@mod) { @forum_post.update!(is_deleted: true) }
-        post_auth undelete_forum_post_path(@forum_post), @mod
-        assert_redirected_to(forum_post_path(@forum_post))
-        assert_equal(false, @forum_post.reload.is_deleted?)
+        @forum_reply = as(@user) { create(:forum_post, topic: @forum_topic, creator: @user, is_deleted: true) }
+        post_auth undelete_forum_post_path(@forum_reply), @mod
+
+        assert_redirected_to(@forum_reply)
+        assert_equal(false, @forum_reply.reload.is_deleted?)
       end
 
       should "not allow users to undelete their own posts" do
-        as(@mod) { @forum_post.update!(is_deleted: true) }
-        post_auth undelete_forum_post_path(@forum_post), @user
+        @forum_reply = as(@user) { create(:forum_post, topic: @forum_topic, creator: @user, is_deleted: true) }
+        post_auth undelete_forum_post_path(@forum_reply), @user
+
         assert_response 403
-        assert_equal(true, @forum_post.reload.is_deleted?)
+        assert_equal(true, @forum_reply.reload.is_deleted?)
+      end
+
+      should "not allow undeleting posts in deleted topics" do
+        @forum_reply = as(@user) { create(:forum_post, topic: @forum_topic, creator: @user) }
+        as(@user) { @forum_topic.update!(is_deleted: true) }
+        assert_equal(true, @forum_reply.reload.is_deleted?)
+
+        post_auth undelete_forum_post_path(@forum_reply), @mod
+
+        assert_response 403
+        assert_equal(true, @forum_reply.reload.is_deleted?)
       end
     end
   end
diff --git a/test/functional/forum_topics_controller_test.rb b/test/functional/forum_topics_controller_test.rb
index d59707331..9a7fa3c6e 100644
--- a/test/functional/forum_topics_controller_test.rb
+++ b/test/functional/forum_topics_controller_test.rb
@@ -306,11 +306,13 @@ class ForumTopicsControllerTest < ActionDispatch::IntegrationTest
         end
       end
 
-      should "destroy the topic and any associated posts" do
+      should "mark the topic and all posts as deleted" do
         delete_auth forum_topic_path(@forum_topic), @mod
-        assert_redirected_to(forum_topic_path(@forum_topic))
-        @forum_topic.reload
-        assert(@forum_topic.is_deleted?)
+
+        assert_redirected_to(@forum_topic)
+        assert_equal(true, @forum_topic.reload.is_deleted?)
+        assert_equal(true, @forum_topic.original_post.is_deleted?)
+        assert_equal(true, @forum_topic.forum_posts.all?(&:is_deleted?))
       end
     end
 
@@ -321,11 +323,13 @@ class ForumTopicsControllerTest < ActionDispatch::IntegrationTest
         end
       end
 
-      should "restore the topic" do
+      should "mark the topic and all posts as undeleted" do
         post_auth undelete_forum_topic_path(@forum_topic), @mod
-        assert_redirected_to(forum_topic_path(@forum_topic))
-        @forum_topic.reload
-        assert(!@forum_topic.is_deleted?)
+
+        assert_redirected_to(@forum_topic)
+        assert_equal(false, @forum_topic.reload.is_deleted?)
+        assert_equal(false, @forum_topic.original_post.is_deleted?)
+        assert_equal(false, @forum_topic.forum_posts.all?(&:is_deleted?))
       end
     end
   end