diff --git a/README.md b/README.md
index d990fb83..9d68fada 100644
--- a/README.md
+++ b/README.md
@@ -499,3 +499,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 92. Reverse Linked List II                                  | [Link](https://leetcode.com/problems/reverse-linked-list-ii/)                                  | [Link](./lib/medium/92_reverse_linked_list_ii.rb)                                  |
 | 95. Unique Binary Search Trees II                           | [Link](https://leetcode.com/problems/unique-binary-search-trees-ii/)                           | [Link](./lib/medium/95_unique_binary_search_trees_ii.rb)                           |
 | 96. Unique Binary Search Trees                              | [Link](https://leetcode.com/problems/unique-binary-search-trees/)                              | [Link](./lib/medium/96_unique_binary_search_trees.rb)                              |
+| 98. Validate Binary Search Tree                             | [Link](https://leetcode.com/problems/validate-binary-search-tree/)                             | [Link](./lib/medium/98_validate_binary_search_tree.rb)                             |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index e05d6fff..c67f7690 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 = '6.3.3'
+  s.version = '6.3.4'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'
diff --git a/lib/medium/98_validate_binary_search_tree.rb b/lib/medium/98_validate_binary_search_tree.rb
new file mode 100644
index 00000000..52a759a7
--- /dev/null
+++ b/lib/medium/98_validate_binary_search_tree.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/validate-binary-search-tree/
+# @param {TreeNode} root
+# @return {Boolean}
+def is_valid_bst(root)
+  is_bst(root, nil, nil)
+end
+
+private
+
+# @param {TreeNode} root
+# @param {TreeNode} left
+# @param {TreeNode} right
+# @return {Boolean}
+def is_bst(node, left, right)
+  return true unless node
+
+  return false if !left.nil? && node.val <= left.val
+
+  return false if !right.nil? && node.val >= right.val
+
+  is_bst(node.left, left, node) && is_bst(node.right, node, right)
+end
diff --git a/test/medium/test_98_validate_binary_search_tree.rb b/test/medium/test_98_validate_binary_search_tree.rb
new file mode 100644
index 00000000..09918881
--- /dev/null
+++ b/test/medium/test_98_validate_binary_search_tree.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/common/binary_tree'
+require_relative '../../lib/medium/98_validate_binary_search_tree'
+require 'minitest/autorun'
+
+class ValidateBinarySearchTreeTest < ::Minitest::Test
+  def test_default
+    assert(
+      is_valid_bst(
+        ::TreeNode.new(
+          2,
+          ::TreeNode.new(1),
+          ::TreeNode.new(3)
+        )
+      )
+    )
+    assert(
+      !is_valid_bst(
+        ::TreeNode.new(
+          5,
+          ::TreeNode.new(1),
+          ::TreeNode.new(
+            4,
+            ::TreeNode.new(3),
+            ::TreeNode.new(6)
+          )
+        )
+      )
+    )
+  end
+end