diff --git a/fastapi/encoders.py b/fastapi/encoders.py
index 451ea0760..814d900a1 100644
--- a/fastapi/encoders.py
+++ b/fastapi/encoders.py
@@ -49,7 +49,8 @@ def decimal_encoder(dec_value: Decimal) -> Union[int, float]:
     >>> decimal_encoder(Decimal("1"))
     1
     """
-    if dec_value.as_tuple().exponent >= 0:  # type: ignore[operator]
+    exponent = dec_value.as_tuple().exponent
+    if isinstance(exponent, int) and exponent >= 0:
         return int(dec_value)
     else:
         return float(dec_value)
diff --git a/tests/test_jsonable_encoder.py b/tests/test_jsonable_encoder.py
index 1906d6bf1..29fb90aeb 100644
--- a/tests/test_jsonable_encoder.py
+++ b/tests/test_jsonable_encoder.py
@@ -3,6 +3,7 @@ from dataclasses import dataclass
 from datetime import datetime, timezone
 from decimal import Decimal
 from enum import Enum
+from math import isinf, isnan
 from pathlib import PurePath, PurePosixPath, PureWindowsPath
 from typing import Optional
 
@@ -303,6 +304,20 @@ def test_decimal_encoder_int():
     assert jsonable_encoder(data) == {"value": 2}
 
 
+@needs_pydanticv2
+def test_decimal_encoder_nan():
+    data = {"value": Decimal("NaN")}
+    assert isnan(jsonable_encoder(data)["value"])
+
+
+@needs_pydanticv2
+def test_decimal_encoder_infinity():
+    data = {"value": Decimal("Infinity")}
+    assert isinf(jsonable_encoder(data)["value"])
+    data = {"value": Decimal("-Infinity")}
+    assert isinf(jsonable_encoder(data)["value"])
+
+
 def test_encode_deque_encodes_child_models():
     class Model(BaseModel):
         test: str