diff --git a/Lib/test/test_codecencodings_iso2022.py b/Lib/test/test_codecencodings_iso2022.py
index 027dbecc6134df..4515997dfa02a2 100644
--- a/Lib/test/test_codecencodings_iso2022.py
+++ b/Lib/test/test_codecencodings_iso2022.py
@@ -15,6 +15,10 @@ class Test_ISO2022_JP(multibytecodec_support.TestBase, unittest.TestCase):
     tstring = multibytecodec_support.load_teststring('iso2022_jp')
     codectests = COMMON_CODEC_TESTS + (
         (b'ab\x1BNdef', 'replace', 'ab\x1BNdef'),
+        # there is no G1 in this encoding
+        (b'ab\x1B$)Bdef', 'strict', None),
+        (b'ab\x1B)Jdef', 'strict', None),
+        (b'ab\x1B)Bdef', 'strict', None),
     )
 
 class Test_ISO2022_JP2(multibytecodec_support.TestBase, unittest.TestCase):
@@ -22,6 +26,14 @@ class Test_ISO2022_JP2(multibytecodec_support.TestBase, unittest.TestCase):
     tstring = multibytecodec_support.load_teststring('iso2022_jp')
     codectests = COMMON_CODEC_TESTS + (
         (b'ab\x1BNdef', 'replace', 'abdef'),
+        # a charset can only be designated to its own plane
+        (b'\x1B.A\x1BNi', 'strict', '\xe9'),
+        (b'\x1B$(C\x30\x21\x1B(B', 'strict', '\uac00'),
+        (b'ab\x1B(Adef', 'strict', None),
+        (b'ab\x1B(Fdef', 'strict', None),
+        (b'ab\x1B.J\x1BNdef', 'strict', None),
+        (b'ab\x1B.Bdef', 'strict', None),
+        (b'ab\x1B$)Cdef', 'strict', None),
     )
 
 class Test_ISO2022_JP3(multibytecodec_support.TestBase, unittest.TestCase):
@@ -75,6 +87,12 @@ class Test_ISO2022_KR(multibytecodec_support.TestBase, unittest.TestCase):
     tstring = multibytecodec_support.load_teststring('iso2022_kr')
     codectests = COMMON_CODEC_TESTS + (
         (b'ab\x1BNdef', 'replace', 'ab\x1BNdef'),
+        # KS X 1001 can only be designated to G1
+        (b'\x1B$)C\x0E\x30\x21\x0Fdef', 'strict', '\uac00def'),
+        (b'ab\x1B$(Cdefg', 'strict', None),
+        (b'ab\x1B$Cdefg', 'strict', None),
+        (b'ab\x1B)Bdefg', 'strict', None),
+        (b'ab\x1B$(Cdefg', 'replace', 'ab\ufffddefg'),
     )
 
     # iso2022_kr.txt cannot be used to test "chunk coding": the escape
diff --git a/Misc/NEWS.d/next/Library/2026-10-22-00-30-04.gh-issue-160187.9dcj2B.rst b/Misc/NEWS.d/next/Library/2026-10-22-00-30-04.gh-issue-160187.9dcj2B.rst
new file mode 100644
index 00000000000000..74ee6c1fa7bbb9
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-22-00-30-04.gh-issue-160187.9dcj2B.rst
@@ -0,0 +1,4 @@
+Fix the ISO-2022 decoders (``iso2022_jp``, ``iso2022_kr`` and related)
+accepting escape sequences that designate a character set to the wrong code
+element (G0, G1 or G2).  Such sequences now raise :exc:`UnicodeDecodeError`
+instead of producing wrong text or :exc:`RuntimeError`.
diff --git a/Modules/cjkcodecs/_codecs_iso2022.c b/Modules/cjkcodecs/_codecs_iso2022.c
index 662c2e1188e817..0a017cfc445fc9 100644
--- a/Modules/cjkcodecs/_codecs_iso2022.c
+++ b/Modules/cjkcodecs/_codecs_iso2022.c
@@ -382,17 +382,22 @@ iso2022processesc(const MultibyteCodec *codec, MultibyteCodec_State *state,
         return esclen;
     }
 
-    /* raise error when the charset is not designated for this encoding */
+    /* raise error when the charset is not designated to this plane for
+     * this encoding (ASCII is only ever designated to G0) */
     if (charset != CHARSET_ASCII) {
         const struct iso2022_designation *dsg;
 
         for (dsg = CONFIG_DESIGNATIONS; dsg->mark; dsg++) {
-            if (dsg->mark == charset)
+            if (dsg->mark == charset && dsg->plane == designation) {
                 break;
+            }
         }
         if (!dsg->mark)
             return esclen;
     }
+    else if (designation != 0) {
+        return esclen;
+    }
 
     STATE_SETG(designation, charset);
     *inleft -= esclen;