diff --git a/middleman-core/lib/middleman-core/extension.rb b/middleman-core/lib/middleman-core/extension.rb
index f4248657..cff75105 100644
--- a/middleman-core/lib/middleman-core/extension.rb
+++ b/middleman-core/lib/middleman-core/extension.rb
@@ -81,6 +81,12 @@ module Middleman
     #   @return [Symbol] the name this extension is registered under. This is the symbol used to activate the extension.
     class_attribute :ext_name, instance_reader: false, instance_writer: false
 
+    # @!attribute resource_list_manipulator_priority
+    #   @!scope class
+    #   @return [Numeric] the priority for this extension's `manipulate_resource_list` method, if it has one.
+    #   @see Middleman::Sitemap::Store#register_resource_list_manipulator
+    class_attribute :resource_list_manipulator_priority, instance_reader: false, instance_writer: false
+
     class << self
       # @api private
       # @return [Middleman::Configuration::ConfigurationManager] The defined options for this extension.
@@ -150,7 +156,7 @@ module Middleman
       # @return [void]
       def activated_extension(instance)
         name = instance.class.ext_name
-        return unless @_extension_activation_callbacks && @_extension_activation_callbacks.has_key?(name)
+        return unless @_extension_activation_callbacks && @_extension_activation_callbacks.key?(name)
         @_extension_activation_callbacks[name].each do |block|
           block.arity == 1 ? block.call(instance) : block.call
         end
@@ -276,7 +282,7 @@ module Middleman
 
         # rubocop:disable IfUnlessModifier
         if ext.respond_to?(:manipulate_resource_list)
-          ext.app.sitemap.register_resource_list_manipulator(ext.class.ext_name, ext)
+          ext.app.sitemap.register_resource_list_manipulator(ext.class.ext_name, ext, ext.class.resource_list_manipulator_priority)
         end
       end
     end
diff --git a/middleman-core/lib/middleman-core/extensions/directory_indexes.rb b/middleman-core/lib/middleman-core/extensions/directory_indexes.rb
index 5c14f723..68e9f958 100644
--- a/middleman-core/lib/middleman-core/extensions/directory_indexes.rb
+++ b/middleman-core/lib/middleman-core/extensions/directory_indexes.rb
@@ -1,5 +1,9 @@
 # Directory Indexes extension
 class Middleman::Extensions::DirectoryIndexes < ::Middleman::Extension
+  # This should run after most other sitemap manipulators so that it
+  # gets a chance to modify any new resources that get added.
+  self.resource_list_manipulator_priority = 100
+
   # Update the main sitemap resource list
   # @return [void]
   def manipulate_resource_list(resources)
diff --git a/middleman-core/lib/middleman-core/sitemap/store.rb b/middleman-core/lib/middleman-core/sitemap/store.rb
index 816d94d5..992cae00 100644
--- a/middleman-core/lib/middleman-core/sitemap/store.rb
+++ b/middleman-core/lib/middleman-core/sitemap/store.rb
@@ -62,9 +62,19 @@ module Middleman
       #
       # @param [Symbol] name Name of the manipulator for debugging
       # @param [#manipulate_resource_list] manipulator Resource list manipulator
+      # @param [Numeric] priority Sets the order of this resource list manipulator relative to the rest. By default this is 50, and manipulators run in the order they are registered, but if a priority is provided then this will run ahead of or behind other manipulators.
       # @return [void]
-      def register_resource_list_manipulator(name, manipulator, *)
-        @resource_list_manipulators << [name, manipulator]
+      def register_resource_list_manipulator(name, manipulator, priority=50)
+        # The third argument used to be a boolean - handle those who still pass one
+        priority = 50 unless priority.is_a? Numeric
+        @resource_list_manipulators << [name, manipulator, priority]
+        # The index trick is used so that the sort is stable - manipulators with the same priority
+        # will always be ordered in the same order as they were registered.
+        n = 0
+        @resource_list_manipulators = @resource_list_manipulators.sort_by do |m|
+          n += 1
+          [m[2], n]
+        end
         rebuild_resource_list!(:registered_new)
       end
 
@@ -212,8 +222,8 @@ module Middleman
 
           @app.logger.debug '== Rebuilding resource list'
 
-          @resources = @resource_list_manipulators.reduce([]) do |result, (_, inst)|
-            newres = inst.manipulate_resource_list(result)
+          @resources = @resource_list_manipulators.reduce([]) do |result, (_, manipulator, _)|
+            newres = manipulator.manipulate_resource_list(result)
 
             # Reset lookup cache
             reset_lookup_cache!