diff --git a/string_utils/validation.py b/string_utils/validation.py
index 9124cbb..90eed42 100644
--- a/string_utils/validation.py
+++ b/string_utils/validation.py
@@ -68,7 +68,8 @@ def is_isbn_10(self) -> bool:
 
             try:
                 for index, digit in enumerate(self.input_string):
-                    product += int(digit) * (index + 1)
+                    value = 10 if index == 9 and digit == 'X' else int(digit)
+                    product += value * (index + 1)
 
                 return product % 11 == 0
 
diff --git a/tests/test_is_isbn_10.py b/tests/test_is_isbn_10.py
index d440e8c..925289c 100644
--- a/tests/test_is_isbn_10.py
+++ b/tests/test_is_isbn_10.py
@@ -1,7 +1,7 @@
 from unittest import TestCase
 
 from string_utils.errors import InvalidInputError
-from string_utils.validation import is_isbn_10
+from string_utils.validation import is_isbn, is_isbn_10
 
 
 class IsISBN10TestCase(TestCase):
@@ -26,6 +26,20 @@ def test_returns_true_for_valid_isbn_10(self):
         self.assertTrue(is_isbn_10('0718079183'))
         self.assertTrue(is_isbn_10('8830102180'))
 
+    def test_x_check_digit(self):
+        for isbn in ('080442957X', '0-8044-2957-X', '097522980X'):
+            with self.subTest(isbn=isbn):
+                self.assertTrue(is_isbn_10(isbn))
+                self.assertTrue(is_isbn(isbn))
+        self.assertTrue(is_isbn_10('080442957X', normalize=False))
+        self.assertFalse(is_isbn_10('0-8044-2957-X', normalize=False))
+
+    def test_invalid_x_check_digit(self):
+        for isbn in ('080442958X', 'X804429570', '08044295XX', '080442957x'):
+            with self.subTest(isbn=isbn):
+                self.assertFalse(is_isbn_10(isbn))
+                self.assertFalse(is_isbn(isbn))
+
     def test_dashes_are_not_considered_by_default(self):
         # isbn numbers have been taken from actual books on amazon
         self.assertTrue(is_isbn_10('150-6715214'))