diff --git a/src/parsy/__init__.py b/src/parsy/__init__.py
index d559de2..c2a0844 100644
--- a/src/parsy/__init__.py
+++ b/src/parsy/__init__.py
@@ -545,6 +545,27 @@ def string_parser(stream, index):
     return string_parser
 
 
+def bstring(expected_bytes: bytes, transform: Callable[[bytes], bytes] = noop) -> Parser:
+    """Returns a parser that expects the ``expected_bytes`` and produces
+    that bytestring value.
+
+    Optionally, a transform function can be passed, which will be used on both
+    the expected bytestring and tested bytestring.
+    """
+
+    blen = len(expected_bytes)
+    transformed_b = transform(expected_bytes)
+
+    @Parser
+    def bytes_parser(stream, index):
+        if transform(stream[index : index + blen]) == transformed_b:
+            return Result.success(index + blen, transformed_b)
+        else:
+            return Result.failure(index, expected_bytes)
+
+    return bytes_parser
+
+
 def regex(exp: str, flags=0, group: int | str | tuple = 0) -> Parser:
     """
     Returns a parser that expects the given ``exp``, and produces the
diff --git a/tests/test_parsy.py b/tests/test_parsy.py
index a3908b1..c535047 100644
--- a/tests/test_parsy.py
+++ b/tests/test_parsy.py
@@ -9,6 +9,7 @@
     ParseError,
     alt,
     any_char,
+    bstring,
     char_from,
     decimal_digit,
     digit,
@@ -53,6 +54,19 @@ def test_string_transform_2(self):
 
         self.assertRaises(ParseError, parser.parse, "dog")
 
+    def test_bstring(self):
+        parser = bstring(b"x")
+        self.assertEqual(parser.parse(b"x"), b"x")
+
+        self.assertRaises(ParseError, parser.parse, b"y")
+
+    def test_bstring_transform(self):
+        parser = bstring(b"x", transform=lambda s: s.lower())
+        self.assertEqual(parser.parse(b"x"), b"x")
+        self.assertEqual(parser.parse(b"X"), b"x")
+
+        self.assertRaises(ParseError, parser.parse, b"y")
+
     def test_regex_str(self):
         parser = regex(r"[0-9]")