diff --git a/Gemfile b/Gemfile
index 77a87d0d..57069f0e 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,6 +4,7 @@ source 'https://rubygems.org'
 
 gem 'minitest', '5.18.0'
 gem 'open-uri', '0.3.0'
+gem 'prime', '0.1.2'
 gem 'rake', '12.3.3'
 gem 'rubocop', '1.7.0'
 gem 'simplecov', '0.22.0'
diff --git a/README.md b/README.md
index 046bd977..9740a60b 100644
--- a/README.md
+++ b/README.md
@@ -533,3 +533,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 189. Rotate Array                                               | [Link](https://leetcode.com/problems/rotate-array/)                                               | [Link](./lib/medium/189_rotate_array.rb)                                               | [Link](./test/medium/test_189_rotate_array.rb)                                               |
 | 199. Binary Tree Right Side View                                | [Link](https://leetcode.com/problems/binary-tree-right-side-view/)                                | [Link](./lib/medium/199_binary_tree_right_side_view.rb)                                | [Link](./test/medium/test_199_binary_tree_right_side_view.rb)                                |
 | 200. Number of Islands                                          | [Link](https://leetcode.com/problems/number-of-islands/)                                          | [Link](./lib/medium/200_number_of_islands.rb)                                          | [Link](./test/medium/test_200_number_of_islands.rb)                                          |
+| 204. Count Primes                                               | [Link](https://leetcode.com/problems/count-primes/)                                               | [Link](./lib/medium/204_count_primes.rb)                                               | [Link](./test/medium/test_204_count_primes.rb)                                               |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index bf6c2578..335321dc 100644
--- a/leetcode-ruby.gemspec
+++ b/leetcode-ruby.gemspec
@@ -5,7 +5,7 @@ require 'English'
 ::Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.0'
   s.name = 'leetcode-ruby'
-  s.version = '6.6.7'
+  s.version = '6.6.8'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/medium/204_count_primes.rb b/lib/medium/204_count_primes.rb
new file mode 100644
index 00000000..a08afd4a
--- /dev/null
+++ b/lib/medium/204_count_primes.rb
@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+require 'prime'
+
+# https://leetcode.com/problems/count-primes/
+# @param {Integer} n
+# @return {Integer}
+def count_primes(n) = ::Prime.each(n - 1).count
diff --git a/test/medium/test_204_count_primes.rb b/test/medium/test_204_count_primes.rb
new file mode 100644
index 00000000..25e573bf
--- /dev/null
+++ b/test/medium/test_204_count_primes.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/medium/204_count_primes'
+require 'minitest/autorun'
+
+class CountPrimesTest < ::Minitest::Test
+  def test_default_one = assert_equal(4, count_primes(10))
+
+  def test_default_two = assert_equal(0, count_primes(0))
+
+  def test_default_three = assert_equal(0, count_primes(1))
+end