diff --git a/src/validators/i18n/es.py b/src/validators/i18n/es.py
index c2761708..ecc4597f 100644
--- a/src/validators/i18n/es.py
+++ b/src/validators/i18n/es.py
@@ -135,7 +135,7 @@ def es_nie(value: str, /):
     """
     number_by_letter = {"X": "0", "Y": "1", "Z": "2"}
     # NIE must must start with X Y or Z
-    if value and value[0] in number_by_letter:
+    if value and value[0].upper() in number_by_letter:
         return _nif_nie_validation(value, number_by_letter)
     return False
 
diff --git a/tests/i18n/test_es.py b/tests/i18n/test_es.py
index 55790400..4254484b 100644
--- a/tests/i18n/test_es.py
+++ b/tests/i18n/test_es.py
@@ -63,6 +63,21 @@ def test_returns_true_on_valid_nie(value: str):
     assert es_nie(value)
 
 
+@pytest.mark.parametrize("validate", [es_nie, es_doi])
+@pytest.mark.parametrize("value", ["X0095892M", "Y2584969J", "Z2915723H"])
+def test_nie_accepts_lowercase_prefix(validate: Callable[[str], object], value: str):
+    """Accept lowercase NIE prefixes consistently with NIF and CIF letters."""
+    assert validate(value[0].lower() + value[1:]) is True
+    assert validate(value.lower()) is True
+
+
+@pytest.mark.parametrize("validate", [es_nie, es_doi])
+@pytest.mark.parametrize("value", ["x0095892x", "y2584969x", "z2915723x"])
+def test_lowercase_nie_still_checks_checksum(validate: Callable[[str], object], value: str):
+    """Do not accept an invalid checksum after normalizing the prefix."""
+    assert isinstance(validate(value), ValidationError)
+
+
 @pytest.mark.parametrize(
     ("value",),
     [