diff --git a/Lib/email/contentmanager.py b/Lib/email/contentmanager.py
index 11d1536db27d79c..dfda3519d585e82 100644
--- a/Lib/email/contentmanager.py
+++ b/Lib/email/contentmanager.py
@@ -186,11 +186,12 @@ def set_text_content(msg, string, subtype="plain", charset='utf-8', cte=None,
                      disposition=None, filename=None, cid=None,
                      params=None, headers=None):
     _prepare_set(msg, 'text', subtype, headers)
+
+    cs = email.charset.Charset(charset)
+    charset = cs.output_charset
     cte, payload = _encode_text(string, charset, cte, msg.policy)
     msg.set_payload(payload)
-    msg.set_param('charset',
-                  email.charset.ALIASES.get(charset, charset),
-                  replace=True)
+    msg.set_param('charset', charset, replace=True)
     msg['Content-Transfer-Encoding'] = cte
     _finalize_set(msg, disposition, filename, cid, params)
 raw_data_manager.add_set_handler(str, set_text_content)
diff --git a/Lib/test/test_email/test_contentmanager.py b/Lib/test/test_email/test_contentmanager.py
index dceb54f15e48f4e..5904ea664023689 100644
--- a/Lib/test/test_email/test_contentmanager.py
+++ b/Lib/test/test_email/test_contentmanager.py
@@ -342,6 +342,46 @@ def test_set_text_charset_latin_1(self):
         self.assertEqual(m.get_payload(decode=True).decode('utf-8'), content)
         self.assertEqual(m.get_content(), content)
 
+    def test_set_text_charset_shift_jis(self):
+        m = self._make_message()
+        content = "\u65e5\u672c\u8a9e\n"
+        raw_data_manager.set_content(m, content, charset='shift_jis')
+        self.assertEqual(m['Content-Type'], 'text/plain; charset="iso-2022-jp"')
+        self.assertEqual(m.get_payload(decode=True), content.encode('iso-2022-jp'))
+        self.assertEqual(m.get_content(), content)
+        self.assertEqual(str(m), textwrap.dedent("""\
+            Content-Type: text/plain; charset="iso-2022-jp"
+            Content-Transfer-Encoding: 7bit
+
+            \x1b$BF|K\\8l\x1b(B
+            """))
+        self.assertEqual(bytes(m), textwrap.dedent("""\
+            Content-Type: text/plain; charset="iso-2022-jp"
+            Content-Transfer-Encoding: 7bit
+
+            \u65e5\u672c\u8a9e
+            """).encode('iso-2022-jp'))
+
+    def test_set_text_charset_euc_jp(self):
+        m = self._make_message()
+        content = "\u65e5\u672c\u8a9e\n"
+        raw_data_manager.set_content(m, content, charset='euc-jp')
+        self.assertEqual(m['Content-Type'], 'text/plain; charset="iso-2022-jp"')
+        self.assertEqual(m.get_payload(decode=True), content.encode('iso-2022-jp'))
+        self.assertEqual(m.get_content(), content)
+        self.assertEqual(str(m), textwrap.dedent("""\
+            Content-Type: text/plain; charset="iso-2022-jp"
+            Content-Transfer-Encoding: 7bit
+
+            \x1b$BF|K\\8l\x1b(B
+            """))
+        self.assertEqual(bytes(m), textwrap.dedent("""\
+            Content-Type: text/plain; charset="iso-2022-jp"
+            Content-Transfer-Encoding: 7bit
+
+            \u65e5\u672c\u8a9e
+            """).encode('iso-2022-jp'))
+
     def test_set_text_plain_long_line_heuristics(self):
         m = self._make_message()
         content = ("Simple but long message that is over 78 characters"
diff --git a/Misc/NEWS.d/next/Library/2026-06-09-12-00-00.gh-issue-150771.K7mNx2.rst b/Misc/NEWS.d/next/Library/2026-06-09-12-00-00.gh-issue-150771.K7mNx2.rst
new file mode 100644
index 000000000000000..2965176d438172e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-09-12-00-00.gh-issue-150771.K7mNx2.rst
@@ -0,0 +1,4 @@
+Fix serialization of :mod:`email` messages using the ``shift_jis`` or
+``euc-jp`` charsets: ``set_content()`` now encodes the payload using
+the output charset, so ``str(m)`` no longer raises
+:exc:`UnicodeEncodeError`.