diff --git a/strings/longest_substring_without_repeating_characters.py b/strings/longest_substring_without_repeating_characters.py
new file mode 100644
index 000000000000..64d3fcc366cc
--- /dev/null
+++ b/strings/longest_substring_without_repeating_characters.py
@@ -0,0 +1,57 @@
+"""
+Longest substring without repeating characters Problem Statement: Given a string,
+find the length of longest substring without repeating characters.
+Example: The longest substring without repeating characters in the string
+"abcabcbb" is "abc". Therefore, algorithm should return 3 as the output.
+"""
+
+
+def longest_substring_without_repeating_characters(string: str) -> int:
+    """
+    Finds the length of longest substring without repeating characters.
+    >>> longest_substring_without_repeating_characters("abcabcbb")
+    3
+    >>> longest_substring_without_repeating_characters("bbbbb")
+    1
+    >>> longest_substring_without_repeating_characters("pwwkew")
+    3
+    >>> longest_substring_without_repeating_characters("")
+    0
+    >>> longest_substring_without_repeating_characters("a")
+    1
+    >>> longest_substring_without_repeating_characters("heaesgliengs")
+    6
+    >>> longest_substring_without_repeating_characters("general kenobi")
+    10
+    >>> longest_substring_without_repeating_characters("that's what she said")
+    7
+    >>> longest_substring_without_repeating_characters(69)
+    Traceback (most recent call last):
+        ...
+    ValueError: longest_substring_without_repeating_characters() takes a string as input
+    """
+
+    if not isinstance(string, str):
+        raise ValueError(
+            "longest_substring_without_repeating_characters() takes a string as input"
+        )
+
+    char_set: set[str] = set()
+    start = 0
+    max_length = 0
+
+    for end in range(len(string)):
+        while string[end] in char_set:
+            char_set.remove(string[start])
+            start += 1
+        char_set.add(string[end])
+        if max_length < end - start + 1:
+            max_length = end - start + 1
+
+    return max_length
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()