diff --git a/README.md b/README.md
index 60541ad5..046bd977 100644
--- a/README.md
+++ b/README.md
@@ -532,3 +532,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 187. Repeated DNA Sequences                                     | [Link](https://leetcode.com/problems/repeated-dna-sequences/)                                     | [Link](./lib/medium/187_repeated_dna_sequences.rb)                                     | [Link](./test/medium/test_187_repeated_dna_sequences.rb)                                     |
 | 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)                                          |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index 666d482d..bf6c2578 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 = '6.6.6'
+  s.version = '6.6.7'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/medium/200_number_of_islands.rb b/lib/medium/200_number_of_islands.rb
new file mode 100644
index 00000000..3dd17eab
--- /dev/null
+++ b/lib/medium/200_number_of_islands.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/number-of-islands/
+# @param {Character[][]} grid
+# @return {Integer}
+def num_islands(grid)
+  result = 0
+  (0...grid.size).each do |i|
+    (0...grid.first.size).each do |j|
+      next unless grid[i][j] == '1'
+
+      result += 1
+
+      dfs200(grid, i, j)
+    end
+  end
+
+  result
+end
+
+private
+
+# @param {Character[][]} grid
+# @param {Integer} i
+# @param {Integer} j
+# @return {Void}
+def dfs200(grid, i, j)
+  return if i.negative? || j.negative? || i >= grid.size || j >= grid.first.size || grid[i][j] == '0'
+
+  grid[i][j] = '0'
+
+  dfs200(grid, i + 1, j)
+  dfs200(grid, i - 1, j)
+  dfs200(grid, i, j + 1)
+  dfs200(grid, i, j - 1)
+end
diff --git a/test/medium/test_200_number_of_islands.rb b/test/medium/test_200_number_of_islands.rb
new file mode 100644
index 00000000..dcf4d9c7
--- /dev/null
+++ b/test/medium/test_200_number_of_islands.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/medium/200_number_of_islands'
+require 'minitest/autorun'
+
+class NumberOfIslandsTest < ::Minitest::Test
+  def test_default_one
+    assert_equal(
+      1,
+      num_islands(
+        [
+          %w[1 1 1 1 0],
+          %w[1 1 0 1 0],
+          %w[1 1 0 0 0],
+          %w[0 0 0 0 0]
+        ]
+      )
+    )
+  end
+
+  def test_default_two
+    assert_equal(
+      3,
+      num_islands(
+        [
+          %w[1 1 0 0 0],
+          %w[1 1 0 0 0],
+          %w[0 0 1 0 0],
+          %w[0 0 0 1 1]
+        ]
+      )
+    )
+  end
+end