diff --git a/changelog/fix_a_false_positive_for_rails_action_controller_flash_before_render.md b/changelog/fix_a_false_positive_for_rails_action_controller_flash_before_render.md
new file mode 100644
index 0000000000..02897e95b2
--- /dev/null
+++ b/changelog/fix_a_false_positive_for_rails_action_controller_flash_before_render.md
@@ -0,0 +1 @@
+* [#843](https://github.com/rubocop/rubocop-rails/issues/843): Fix a false positive for `Rails/ActionControllerFlashBeforeRender` when using `flash` in multiline `if` branch before `redirect_to`. ([@koic][])
diff --git a/lib/rubocop/cop/rails/action_controller_flash_before_render.rb b/lib/rubocop/cop/rails/action_controller_flash_before_render.rb
index e101de4b3d..9d1f1cd56e 100644
--- a/lib/rubocop/cop/rails/action_controller_flash_before_render.rb
+++ b/lib/rubocop/cop/rails/action_controller_flash_before_render.rb
@@ -69,9 +69,12 @@ def on_send(flash_node)
         def followed_by_render?(flash_node)
           flash_assigment_node = find_ancestor(flash_node, type: :send)
           context = flash_assigment_node
-          context = context.parent if context.parent.if_type?
+          if (if_node = context.each_ancestor(:if).first)
+            context = if_node
+          elsif context.right_siblings.empty?
+            return true
+          end
           context = context.right_siblings
-          return true if context.empty?
 
           context.compact.any? do |node|
             render?(node)
diff --git a/spec/rubocop/cop/rails/action_controller_flash_before_render_spec.rb b/spec/rubocop/cop/rails/action_controller_flash_before_render_spec.rb
index 4772a1a8dd..6cf9dd10e8 100644
--- a/spec/rubocop/cop/rails/action_controller_flash_before_render_spec.rb
+++ b/spec/rubocop/cop/rails/action_controller_flash_before_render_spec.rb
@@ -152,6 +152,50 @@ def create
               end
             RUBY
           end
+
+          it 'registers an offense when using `flash` in multiline `if` branch before `render_to`' do
+            expect_offense(<<~RUBY)
+              class HomeController < #{parent_class}
+                def create
+                  if condition
+                    do_something
+                    flash[:alert] = "msg"
+                    ^^^^^ Use `flash.now` before `render`.
+                  end
+
+                  render :index
+                end
+              end
+            RUBY
+
+            expect_correction(<<~RUBY)
+              class HomeController < #{parent_class}
+                def create
+                  if condition
+                    do_something
+                    flash.now[:alert] = "msg"
+                  end
+
+                  render :index
+                end
+              end
+            RUBY
+          end
+
+          it 'does not register an offense when using `flash` in multiline `if` branch before `redirect_to`' do
+            expect_no_offenses(<<~RUBY)
+              class HomeController < #{parent_class}
+                def create
+                  if condition
+                    do_something
+                    flash[:alert] = "msg"
+                  end
+
+                  redirect_to :index
+                end
+              end
+            RUBY
+          end
         end
       end