diff --git a/DIRECTORY.md b/DIRECTORY.md
index 1dea295..1eb4e23 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -55,6 +55,7 @@
     * [Common Characters](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/common_characters.rb)
     * [Find All Duplicates In An Array](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/find_all_duplicates_in_an_array.rb)
     * [Good Pairs](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/good_pairs.rb)
+    * [Isomorphic Strings](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/isomorphic_strings.rb)
     * [Richest Customer Wealth](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/richest_customer_wealth.rb)
     * [Two Sum](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/two_sum.rb)
     * [Uncommon Words](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/uncommon_words.rb)
diff --git a/data_structures/hash_table/isomorphic_strings.rb b/data_structures/hash_table/isomorphic_strings.rb
new file mode 100644
index 0000000..cc875ab
--- /dev/null
+++ b/data_structures/hash_table/isomorphic_strings.rb
@@ -0,0 +1,64 @@
+# Challenge name: Isomorphic Strings
+#
+# Given two strings s and t, determine if they are isomorphic.
+# Two strings s and t are isomorphic if the characters in s can be replaced to get t.
+# All occurrences of a character must be replaced with another character while preserving the order of characters. 
+# No two characters may map to the same character, but a character may map to itself.
+#
+# Example 1:
+# Input: s = "egg", t = "add"
+# Output: true
+#
+# Example 2:
+# Input: s = "foo", t = "bar"
+# Output: false
+#
+# Example 3:
+# Input: s = "paper", t = "title"
+# Output: true
+#
+# Constraints:
+# 1 <= s.length <= 5 * 104
+# t.length == s.length
+# s and t consist of any valid ascii character.
+
+# Approach 1: Hash Map
+# Time Complexity: O(N)
+# Space Complexity: O(N)
+
+def isomorphic_strings_check(s, t)
+  # store character mappings
+  map = {}
+  # store already mapped characters
+  set = []
+
+  (0..s.length - 1).each do |i|
+    # store characters to compare
+    char1 = s[i]
+    char2 = t[i]
+
+    # if char1 is mapped
+    if map[char1] 
+      # return false if char1 is mapped to a different character than already present
+      return false if map[char1] != char2
+    # if char1 is not mapped
+    else
+      # return false if char2 is already mapped to a different character
+      return false if set.include?(char2)
+      # checks passed: add new character map and track that char2 has been mapped
+      map[char1] = char2
+      set << char2
+    end
+
+  end
+  return true
+end
+
+puts isomorphic_strings_check("egg", "add")
+# => true
+
+puts isomorphic_strings_check("foo", "bar")
+# => false
+
+puts isomorphic_strings_check("paper", "title")
+# => true