diff --git a/spacy/lang/ko/__init__.py b/spacy/lang/ko/__init__.py
index 63bc06665..0e02e4a2d 100644
--- a/spacy/lang/ko/__init__.py
+++ b/spacy/lang/ko/__init__.py
@@ -7,7 +7,7 @@ from .lex_attrs import LEX_ATTRS
 from ...language import Language, BaseDefaults
 from ...tokens import Doc
 from ...scorer import Scorer
-from ...symbols import POS
+from ...symbols import POS, X
 from ...training import validate_examples
 from ...util import DummyTokenizer, registry, load_config_from_str
 from ...vocab import Vocab
@@ -57,7 +57,10 @@ class KoreanTokenizer(DummyTokenizer):
         for token, dtoken in zip(doc, dtokens):
             first_tag, sep, eomi_tags = dtoken["tag"].partition("+")
             token.tag_ = first_tag  # stem(어간) or pre-final(선어말 어미)
-            token.pos = TAG_MAP[token.tag_][POS]
+            if token.tag_ in TAG_MAP:
+                token.pos = TAG_MAP[token.tag_][POS]
+            else:
+                token.pos = X
             token.lemma_ = dtoken["lemma"]
         doc.user_data["full_tags"] = [dt["tag"] for dt in dtokens]
         return doc
diff --git a/spacy/tests/lang/ko/test_tokenizer.py b/spacy/tests/lang/ko/test_tokenizer.py
index e6b65dee9..6e06e405e 100644
--- a/spacy/tests/lang/ko/test_tokenizer.py
+++ b/spacy/tests/lang/ko/test_tokenizer.py
@@ -49,6 +49,12 @@ def test_ko_empty_doc(ko_tokenizer):
     assert len(tokens) == 0
 
 
+@pytest.mark.issue(10535)
+def test_ko_tokenizer_unknown_tag(ko_tokenizer):
+    tokens = ko_tokenizer("미닛 리피터")
+    assert tokens[1].pos_ == "X"
+
+
 # fmt: off
 SPACY_TOKENIZER_TESTS = [
     ("있다.", "있다 ."),