diff --git a/Lib/test/test_base64.py b/Lib/test/test_base64.py
--- a/Lib/test/test_base64.py
+++ b/Lib/test/test_base64.py
@@ -22,6 +22,8 @@
            b"YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXpBQkNE"
            b"RUZHSElKS0xNTk9QUVJTVFVWV1hZWjAxMjM0\nNT"
            b"Y3ODkhQCMwXiYqKCk7Ojw+LC4gW117fQ==\n")
+        # Test for bytearray, issue17842
+        eq(base64.encodebytes(bytearray(b'abcd')), b'YWJjZA==\n')
         self.assertRaises(TypeError, base64.encodebytes, "")
 
     def test_decodebytes(self):
@@ -30,6 +32,8 @@
         eq(base64.decodebytes(b"YQ==\n"), b"a")
         eq(base64.decodebytes(b"YWI=\n"), b"ab")
         eq(base64.decodebytes(b"YWJj\n"), b"abc")
+        # Test for bytearray, issue17842
+        eq(base64.decodebytes(bytearray(b"YWJj\n")), b"abc")
         eq(base64.decodebytes(b"YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXpBQkNE"
                                b"RUZHSElKS0xNTk9QUVJTVFVWV1hZWjAxMjM0\nNT"
                                b"Y3ODkhQCMwXiYqKCk7Ojw+LC4gW117fQ==\n"),
@@ -93,6 +97,8 @@
            b"YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXpBQkNE"
            b"RUZHSElKS0xNTk9QUVJTVFVWV1hZWjAxMjM0NT"
            b"Y3ODkhQCMwXiYqKCk7Ojw+LC4gW117fQ==")
+        # Test for bytearray, issue17842
+        eq(base64.standard_b64encode(bytearray(b'abcd')), b'YWJjZA==')
         # Check if passing a str object raises an error
         self.assertRaises(TypeError, base64.standard_b64encode, "")
         self.assertRaises(TypeError, base64.standard_b64encode, b"", altchars="")
@@ -121,6 +127,8 @@
         for data, res in tests.items():
             eq(base64.b64decode(data), res)
             eq(base64.b64decode(data.decode('ascii')), res)
+        # Test for bytearray, issue17842
+        eq(base64.b64decode(bytearray(b"YWJj")), b"abc")
 
         # Test with arbitrary alternative characters
         tests_altchars = {(b'01a*b$cd', b'*$'): b'\xd3V\xbeo\xf7\x1d',
@@ -179,6 +187,8 @@
         eq(base64.b32encode(b'abc'), b'MFRGG===')
         eq(base64.b32encode(b'abcd'), b'MFRGGZA=')
         eq(base64.b32encode(b'abcde'), b'MFRGGZDF')
+        # Test for bytearray, issue17842
+        eq(base64.b32encode(bytearray(b'abcd')), b'MFRGGZA=')
         self.assertRaises(TypeError, base64.b32encode, "")
 
     def test_b32decode(self):
@@ -194,6 +204,8 @@
         for data, res in tests.items():
             eq(base64.b32decode(data), res)
             eq(base64.b32decode(data.decode('ascii')), res)
+        # Test for bytearray, issue17842
+        eq(base64.b32decode(bytearray(b'MFRGG===')), b'abc')
 
     def test_b32decode_casefold(self):
         eq = self.assertEqual
@@ -245,6 +257,8 @@
         eq = self.assertEqual
         eq(base64.b16encode(b'\x01\x02\xab\xcd\xef'), b'0102ABCDEF')
         eq(base64.b16encode(b'\x00'), b'00')
+        # Test for bytearray, issue17842
+        eq(base64.b16encode(bytearray(b'\x01\x02\xab\xcd\xef')), b'0102ABCDEF')
         self.assertRaises(TypeError, base64.b16encode, "")
 
     def test_b16decode(self):
@@ -259,6 +273,8 @@
         # Case fold
         eq(base64.b16decode(b'0102abcdef', True), b'\x01\x02\xab\xcd\xef')
         eq(base64.b16decode('0102abcdef', True), b'\x01\x02\xab\xcd\xef')
+        # Test for bytearray, issue17842
+        eq(base64.b16decode(bytearray(b"0102ABCDEF")), b'\x01\x02\xab\xcd\xef')
 
     def test_decode_nonascii_str(self):
         decode_funcs = (base64.b64decode,