diff --git a/langchain/output_parsers/boolean.py b/langchain/output_parsers/boolean.py
index 40890a9d..96f61252 100644
--- a/langchain/output_parsers/boolean.py
+++ b/langchain/output_parsers/boolean.py
@@ -16,12 +16,12 @@ class BooleanOutputParser(BaseOutputParser[bool]):
 
         """
         cleaned_text = text.strip()
-        if cleaned_text not in (self.true_val, self.false_val):
+        if cleaned_text.upper() not in (self.true_val.upper(), self.false_val.upper()):
             raise ValueError(
                 f"BooleanOutputParser expected output value to either be "
                 f"{self.true_val} or {self.false_val}. Received {cleaned_text}."
             )
-        return cleaned_text == self.true_val
+        return cleaned_text.upper() == self.true_val.upper()
 
     @property
     def _type(self) -> str:
diff --git a/tests/unit_tests/output_parsers/test_boolean_parser.py b/tests/unit_tests/output_parsers/test_boolean_parser.py
index 3daaf610..8804c363 100644
--- a/tests/unit_tests/output_parsers/test_boolean_parser.py
+++ b/tests/unit_tests/output_parsers/test_boolean_parser.py
@@ -12,6 +12,14 @@ def test_boolean_output_parser_parse() -> None:
     result = parser.parse("NO")
     assert result is False
 
+    # Test valid input
+    result = parser.parse("yes")
+    assert result is True
+
+    # Test valid input
+    result = parser.parse("no")
+    assert result is False
+
     # Test invalid input
     try:
         parser.parse("INVALID")