diff --git a/strings/min_window_substring.py b/strings/min_window_substring.py
new file mode 100644
index 000000000..5fec6eb2d
--- /dev/null
+++ b/strings/min_window_substring.py
@@ -0,0 +1,89 @@
+def min_window(search_str: str, target_letters: str) -> str:
+    """
+    Given a string to search, and another string of target char_dict,
+    return the smallest substring of the search string that contains
+    all target char_dict.
+
+    This is somewhat modified from my solution to the problem
+    "Minimum Window Substring" on leetcode.
+    https://leetcode.com/problems/minimum-window-substring/description/
+
+    >>> min_window("Hello World", "lWl")
+    'llo W'
+    >>> min_window("Hello World", "f")
+    ''
+
+    This solution uses a sliding window, alternating between shifting
+    the end of the window right until all target char_dict are contained
+    in the window, and shifting the start of the window right until the
+    window no longer contains every target character.
+
+    Time complexity: O(target_count + search_len) ->
+        The algorithm checks a dictionary at most twice for each character
+        in search_str.
+
+    Space complexity: O(search_len) ->
+        The primary contributor to additional space is the building of a
+        dictionary using the search string.
+    """
+
+    target_count = len(target_letters)
+    search_len = len(search_str)
+
+    # Return if not possible due to string lengths.
+    if search_len < target_count:
+        return ""
+
+    # Build dictionary with counts for each letter in target_letters
+    char_dict = {}
+    for ch in target_letters:
+        if ch not in char_dict:
+            char_dict[ch] = 1
+        else:
+            char_dict[ch] += 1
+
+    # Initialize window
+    window_start = 0
+    window_end = 0
+
+    exists = False
+    min_window_len = search_len + 1
+
+    # Start sliding window algorithm
+    while window_end < search_len:
+        # Slide window end right until all search characters are contained
+        while target_count > 0 and window_end < search_len:
+            cur = search_str[window_end]
+            if cur in char_dict:
+                char_dict[cur] -= 1
+                if char_dict[cur] >= 0:
+                    target_count -= 1
+            window_end += 1
+        temp = window_end - window_start
+
+        # Check if window is the smallest found so far
+        if target_count == 0 and temp < min_window_len:
+            min_window = [window_start, window_end]
+            exists = True
+            min_window_len = temp
+
+        # Slide window start right until a search character exits the window
+        while target_count == 0 and window_start < window_end:
+            cur = search_str[window_start]
+            window_start += 1
+            if cur in char_dict:
+                char_dict[cur] += 1
+                if char_dict[cur] > 0:
+                    break
+        temp = window_end - window_start + 1
+
+        # Check if window is the smallest found so far
+        if temp < min_window_len and target_count == 0:
+            min_window = [window_start - 1, window_end]
+            min_window_len = temp
+        target_count = 1
+
+    if exists:
+        return search_str[min_window[0] : min_window[1]]
+    else:
+        return ""