diff --git a/fastapi/encoders.py b/fastapi/encoders.py
index c9f882d2b..04859f85b 100644
--- a/fastapi/encoders.py
+++ b/fastapi/encoders.py
@@ -73,12 +73,20 @@ def decimal_encoder(dec_value: Decimal) -> int | float:
 
     >>> decimal_encoder(Decimal("NaN"))
     nan
+
+    >>> decimal_encoder(Decimal("sNaN"))
+    nan
     """
     exponent = dec_value.as_tuple().exponent
     if isinstance(exponent, int) and exponent >= 0:
         return int(dec_value)
     else:
-        return float(dec_value)
+        try:
+            return float(dec_value)
+        except ValueError:
+            # Signaling NaN (sNaN) cannot be converted to float directly.
+            # Treat it as a quiet NaN, consistent with Decimal("NaN") behaviour.
+            return float("nan")
 
 
 ENCODERS_BY_TYPE: dict[type[Any], Callable[[Any], Any]] = {
diff --git a/tests/test_jsonable_encoder.py b/tests/test_jsonable_encoder.py
index 8f8bd3fcb..e7efa6512 100644
--- a/tests/test_jsonable_encoder.py
+++ b/tests/test_jsonable_encoder.py
@@ -299,6 +299,14 @@ def test_decimal_encoder_infinity():
     assert isinf(jsonable_encoder(data)["value"])
 
 
+def test_decimal_encoder_signaling_nan():
+    # Decimal("sNaN") (signaling NaN) previously raised ValueError when
+    # decimal_encoder called float() on it. It should now return float("nan")
+    # consistently with Decimal("NaN") behaviour.
+    data = {"value": Decimal("sNaN")}
+    assert isnan(jsonable_encoder(data)["value"])
+
+
 def test_encode_deque_encodes_child_models():
     class Model(BaseModel):
         test: str