diff --git a/Lib/encodings/base64_codec.py b/Lib/encodings/base64_codec.py
index 8e7703b3b6072d6..ce5af465d6d9a4c 100644
--- a/Lib/encodings/base64_codec.py
+++ b/Lib/encodings/base64_codec.py
@@ -29,10 +29,25 @@ def encode(self, input, final=False):
         assert self.errors == 'strict'
         return base64.encodebytes(input)
 
-class IncrementalDecoder(codecs.IncrementalDecoder):
-    def decode(self, input, final=False):
-        assert self.errors == 'strict'
-        return base64.decodebytes(input)
+_alphabet = (b'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+             b'abcdefghijklmnopqrstuvwxyz0123456789+/')
+
+class IncrementalDecoder(codecs.BufferedIncrementalDecoder):
+    def _buffer_decode(self, input, errors, final):
+        assert errors == 'strict'
+        if not final:
+            # Only decode complete groups of four base64 characters and keep
+            # the rest for the next call.  Other bytes, such as newlines and
+            # padding, are not counted.
+            count = len(input) - len(input.translate(None, _alphabet))
+            remainder = count % 4
+            end = len(input)
+            while remainder:
+                end -= 1
+                if input[end] in _alphabet:
+                    remainder -= 1
+            input = input[:end]
+        return (base64.decodebytes(input), len(input))
 
 class StreamWriter(Codec, codecs.StreamWriter):
     charbuffertype = bytes
diff --git a/Lib/encodings/bz2_codec.py b/Lib/encodings/bz2_codec.py
index fd9495e341baee1..a8fb0f3be919c27 100644
--- a/Lib/encodings/bz2_codec.py
+++ b/Lib/encodings/bz2_codec.py
@@ -52,7 +52,7 @@ def decode(self, input, final=False):
         try:
             return self.decompressobj.decompress(input)
         except EOFError:
-            return ''
+            return b''
 
     def reset(self):
         self.decompressobj = bz2.BZ2Decompressor()
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index 37ac57063c35390..194e249f5adeed9 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -3251,6 +3251,34 @@ def test_uu_invalid(self):
         # Missing "begin" line
         self.assertRaises(ValueError, codecs.decode, b"", "uu-codec")
 
+    def test_base64_incremental_decode(self):
+        # gh-71986: Input split in the middle of a group of four characters
+        data = bytes(range(256))
+        encoded = codecs.encode(data, "base64_codec")
+        for size in range(1, 6):
+            with self.subTest(size=size):
+                chunks = [encoded[i:i+size]
+                          for i in range(0, len(encoded), size)]
+                decoded = b"".join(codecs.iterdecode(chunks, "base64_codec"))
+                self.assertEqual(decoded, data)
+
+        decoded = codecs.iterdecode([b"AA", b"AA\r\n", b"A", b"A=="],
+                                    "base64_codec")
+        self.assertEqual(b"".join(decoded), b"\x00\x00\x00\x00")
+
+        decoder = codecs.getincrementaldecoder("base64_codec")()
+        self.assertEqual(decoder.decode(b"AAA"), b"")
+        self.assertRaises(ValueError, decoder.decode, b"", final=True)
+
+    @unittest.skipUnless("bz2_codec" in bytes_transform_encodings,
+                         "Requires bz2 support")
+    def test_bz2_incremental_decode_after_end(self):
+        # gh-71986: The decoder returned str after the end of the stream
+        decoder = codecs.getincrementaldecoder("bz2_codec")()
+        self.assertEqual(decoder.decode(codecs.encode(b"data", "bz2_codec")),
+                         b"data")
+        self.assertEqual(decoder.decode(b"", final=True), b"")
+
 
 # The codec system tries to add notes to exceptions in order to ensure
 # the error mentions the operation being performed and the codec involved.
diff --git a/Misc/NEWS.d/next/Library/2026-10-29-01-13-36.gh-issue-71986.iO8u7u.rst b/Misc/NEWS.d/next/Library/2026-10-29-01-13-36.gh-issue-71986.iO8u7u.rst
new file mode 100644
index 000000000000000..779f031535c2f72
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-29-01-13-36.gh-issue-71986.iO8u7u.rst
@@ -0,0 +1,5 @@
+Fix the incremental decoders of the ``base64_codec`` and ``bz2_codec``
+codecs. The ``base64_codec`` decoder now accepts input split in the middle
+of a group of four characters, and the ``bz2_codec`` decoder now returns
+:class:`bytes` instead of :class:`str` when called after the end of the
+compressed data.