diff --git a/README.md b/README.md
index 752de644..1984f94d 100644
--- a/README.md
+++ b/README.md
@@ -626,6 +626,7 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 817. Linked List Components                                     | [Link](https://leetcode.com/problems/linked-list-components/)                                     | [Link](./lib/medium/817_linked_list_components.rb)                                     | [Link](./test/medium/test_817_linked_list_components.rb)                                     |
 | 823. Binary Trees With Factors                                  | [Link](https://leetcode.com/problems/binary-trees-with-factors/)                                  | [Link](./lib/medium/823_binary_trees_with_factors.rb)                                  | [Link](./test/medium/test_823_binary_trees_with_factors.rb)                                  |
 | 841. Keys and Rooms                                             | [Link](https://leetcode.com/problems/keys-and-rooms/)                                             | [Link](./lib/medium/841_keys_and_rooms.rb)                                             | [Link](./test/medium/test_841_keys_and_rooms.rb)                                             |
+| 852. Peak Index in a Mountain Array                             | [Link](https://leetcode.com/problems/peak-index-in-a-mountain-array/)                             | [Link](./lib/medium/852_peak_index_in_a_mountain_array.rb)                             | [Link](./test/medium/test_852_peak_index_in_a_mountain_array.rb)                             |
 
 ### Hard
 
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index 3a72ce84..44e245c6 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 = '7.6.5'
+  s.version = '7.6.6'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/medium/852_peak_index_in_a_mountain_array.rb b/lib/medium/852_peak_index_in_a_mountain_array.rb
new file mode 100644
index 00000000..2936672d
--- /dev/null
+++ b/lib/medium/852_peak_index_in_a_mountain_array.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/peak-index-in-a-mountain-array/
+# @param {Integer[]} arr
+# @return {Integer}
+def peak_index_in_mountain_array(arr)
+  result = 0
+  max = 0
+  arr.each_with_index do |num, i|
+    next unless num > max
+
+    max = num
+    result = i
+  end
+
+  result
+end
diff --git a/test/medium/test_852_peak_index_in_a_mountain_array.rb b/test/medium/test_852_peak_index_in_a_mountain_array.rb
new file mode 100644
index 00000000..b71153c4
--- /dev/null
+++ b/test/medium/test_852_peak_index_in_a_mountain_array.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/medium/852_peak_index_in_a_mountain_array'
+require 'minitest/autorun'
+
+class PeakIndexInAMountainArrayTest < ::Minitest::Test
+  def test_default_one
+    assert_equal(
+      1,
+      peak_index_in_mountain_array(
+        [0, 1, 0]
+      )
+    )
+  end
+
+  def test_default_two
+    assert_equal(
+      1,
+      peak_index_in_mountain_array(
+        [0, 2, 1, 0]
+      )
+    )
+  end
+
+  def test_default_three
+    assert_equal(
+      1,
+      peak_index_in_mountain_array(
+        [0, 10, 5, 2]
+      )
+    )
+  end
+end