diff --git a/lib/colorls/core.rb b/lib/colorls/core.rb
index 5b385ac..bba18cc 100644
--- a/lib/colorls/core.rb
+++ b/lib/colorls/core.rb
@@ -92,6 +92,13 @@ module ColorLS
 
     def sort_contents
       case @sort
+      when :extension
+        @contents.sort_by! do |f|
+          name = f.name
+          ext = File.extname(name)
+          name = name.chomp(ext) unless ext.empty?
+          [ext, name].map { |s| CLocale.strxfrm(s) }
+        end
       when :time
         @contents.sort_by! { |a| -a.mtime.to_f }
       when :size
diff --git a/lib/colorls/flags.rb b/lib/colorls/flags.rb
index 4ba6663..03b49d2 100644
--- a/lib/colorls/flags.rb
+++ b/lib/colorls/flags.rb
@@ -76,15 +76,15 @@ module ColorLS
       options.on('-t', 'sort by modification time, newest first')                             { @opts[:sort] = :time }
       options.on('-U', 'do not sort; list entries in directory order')                        { @opts[:sort] = false }
       options.on('-S', 'sort by file size, largest first')                                    { @opts[:sort] = :size }
+      options.on('-X', 'sort by file extension')                                              { @opts[:sort] = :extension }
       options.on(
         '--sort=WORD',
-        %w[none time size],
-        'sort by WORD instead of name: none, size (-S), time (-t)'
+        %w[none time size extension],
+        'sort by WORD instead of name: none, size (-S), time (-t), extension (-X)'
       ) do |word|
         @opts[:sort] = case word
                        when 'none' then false
-                       when 'time' then :time
-                       when 'size' then :size
+                       else word.to_sym
                        end
       end
 
diff --git a/spec/color_ls/flags_spec.rb b/spec/color_ls/flags_spec.rb
index 16cf19c..30b1e89 100644
--- a/spec/color_ls/flags_spec.rb
+++ b/spec/color_ls/flags_spec.rb
@@ -85,6 +85,12 @@ RSpec.describe ColorLS::Flags do
     it { is_expected.to match(/a-file.+z-file.+symlinks/) } # sorts results by size
   end
 
+  context 'with --sort=extension flag' do
+    let(:args) { ['--sort=extension', FIXTURES] }
+
+    it { is_expected.to match(/a-file.+symlinks.+z-file.+a.md.+a.txt.+z.txt/) } # sorts results by extension
+  end
+
   context 'with --dirs flag' do
     let(:args) { ['--dirs', FIXTURES] }
 
diff --git a/spec/fixtures/a.md b/spec/fixtures/a.md
new file mode 100644
index 0000000..e69de29
diff --git a/spec/fixtures/a.txt b/spec/fixtures/a.txt
new file mode 100644
index 0000000..e69de29
diff --git a/spec/fixtures/z.txt b/spec/fixtures/z.txt
new file mode 100644
index 0000000..e69de29
diff --git a/zsh/_colorls b/zsh/_colorls
index 79b685f..3777b65 100644
--- a/zsh/_colorls
+++ b/zsh/_colorls
@@ -27,6 +27,7 @@ _arguments -s -S \
   "-t[sort by modification time, newest first]" \
   "-U[do not sort; list entries in directory order]" \
   "-S[sort by file size, largest first]" \
+  "-X[sort by file extension]" \
   "--sort[sort by WORD instead of name: none, size (-S), time (-t)]" \
   "-r[reverse order while sorting]" \
   "--reverse[reverse order while sorting]" \