diff --git a/Lib/email/contentmanager.py b/Lib/email/contentmanager.py
index c0090af716575d7..9815e6a71c076b0 100644
--- a/Lib/email/contentmanager.py
+++ b/Lib/email/contentmanager.py
@@ -231,9 +231,9 @@ def set_bytes_content(msg, data, maintype, subtype, cte='base64',
         data = binascii.b2a_qp(data, istext=False, header=False, quotetabs=True)
         data = data.decode('ascii')
     elif cte == '7bit':
-        data = data.decode('ascii')
+        data = str(data, 'ascii')
     elif cte in ('8bit', 'binary'):
-        data = data.decode('ascii', 'surrogateescape')
+        data = str(data, 'ascii', 'surrogateescape')
     msg.set_payload(data)
     msg['Content-Transfer-Encoding'] = cte
     _finalize_set(msg, disposition, filename, cid, params)
diff --git a/Lib/test/test_email/test_contentmanager.py b/Lib/test/test_email/test_contentmanager.py
index 3115941f8703194..d7e098a0e2b55e7 100644
--- a/Lib/test/test_email/test_contentmanager.py
+++ b/Lib/test/test_email/test_contentmanager.py
@@ -848,6 +848,29 @@ def test_set_content_bytes_cte_7bit(self):
             ASCII-only message.
             """))
 
+    def test_set_byteslike_content_with_unencoded_cte(self):
+        for cte in ('7bit', '8bit', 'binary'):
+            content = (b'ASCII-only message.\n' if cte == '7bit' else
+                       b'non-ASCII \xa7\xff message.\r\n')
+            for buffer_type in (bytes, bytearray, memoryview):
+                with self.subTest(cte=cte, buffer_type=buffer_type):
+                    m = self._make_message()
+                    m.set_content(buffer_type(content),
+                                  maintype='application',
+                                  subtype='octet-stream', cte=cte)
+                    self.assertEqual(m['Content-Transfer-Encoding'], cte)
+                    self.assertEqual(m.get_payload(decode=True), content)
+                    self.assertEqual(m.get_content(), content)
+
+    def test_set_byteslike_non_ascii_with_cte_7bit_raises(self):
+        for buffer_type in (bytes, bytearray, memoryview):
+            with self.subTest(buffer_type=buffer_type):
+                m = self._make_message()
+                with self.assertRaises(UnicodeDecodeError):
+                    m.set_content(buffer_type(b'non-ASCII \xa7\xff message'),
+                                  maintype='application',
+                                  subtype='octet-stream', cte='7bit')
+
     content_object_params = {
         'text_plain': ('content', ()),
         'text_html': ('content', ('html',)),
diff --git a/Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159858.CQw9me.rst b/Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159858.CQw9me.rst
new file mode 100644
index 000000000000000..65fc880be4bba84
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159858.CQw9me.rst
@@ -0,0 +1,3 @@
+Fix :meth:`email.message.EmailMessage.set_content` raising
+:exc:`AttributeError` for :class:`memoryview` payloads with the ``7bit``,
+``8bit``, or ``binary`` content transfer encoding.