diff --git a/app/models/category.rb b/app/models/category.rb
index ad5a662a356..fc9d1377570 100644
--- a/app/models/category.rb
+++ b/app/models/category.rb
@@ -353,6 +353,8 @@ class Category < ActiveRecord::Base
 
       if self.slug.blank?
         errors.add(:slug, :invalid)
+      elsif SiteSetting.slug_generation_method == 'ascii' && !CGI.unescape(self.slug).ascii_only?
+        errors.add(:slug, I18n.t("category.errors.slug_contains_non_ascii_chars"))
       elsif duplicate_slug?
         errors.add(:slug, 'is already in use')
       end
@@ -803,11 +805,7 @@ class Category < ActiveRecord::Base
     return nil if slug_path.size > SiteSetting.max_category_nesting
 
     slug_path.map! do |slug|
-      if SiteSetting.slug_generation_method == "encoded"
-        CGI.escape(slug.downcase)
-      else
-        slug.downcase
-      end
+      CGI.escape(slug.downcase)
     end
 
     query =
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 24fe3a11b7f..18abca279ca 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -704,6 +704,7 @@ en:
       permission_conflict: "Any group that is allowed to access a subcategory must also be allowed to access the parent category. The following groups have access to one of the subcategories, but no access to parent category: %{group_names}."
       disallowed_topic_tags: "This topic has tags not allowed by this category: '%{tags}'"
       disallowed_tags_generic: "This topic has disallowed tags."
+      slug_contains_non_ascii_chars: "contains non-ascii characters"
     cannot_delete:
       uncategorized: "This category is special. It is intended as a holding area for topics that have no category; it cannot be deleted."
       has_subcategories: "Can't delete this category because it has sub-categories."
diff --git a/spec/models/category_spec.rb b/spec/models/category_spec.rb
index 85ba20b150c..d12aa8a66e6 100644
--- a/spec/models/category_spec.rb
+++ b/spec/models/category_spec.rb
@@ -362,6 +362,19 @@ describe Category do
       expect(@cat).to be_valid
       expect(@cat.errors[:slug]).not_to be_present
     end
+
+    context 'if SiteSettings.slug_generation_method = ascii' do
+      before do
+        SiteSetting.slug_generation_method = 'ascii'
+      end
+
+      it 'fails if slug contains non-ascii characters' do
+        c = Fabricate.build(:category, name: "Sem acentuação", slug: "sem-acentuação")
+        expect(c).not_to be_valid
+
+        expect(c.errors[:slug]).to be_present
+      end
+    end
   end
 
   describe 'description_text' do