diff --git a/src/humanize/number.py b/src/humanize/number.py
index 52a5356a..ac16d733 100644
--- a/src/humanize/number.py
+++ b/src/humanize/number.py
@@ -184,7 +184,27 @@ def intcomma(value: NumberOrString, ndigits: int | None = None) -> str:
             if not math.isfinite(float(value)):
                 return _format_not_finite(float(value))
             if "." in value:
-                value = float(value)
+                # Without an explicit rounding request, grouping a decimal
+                # string must not lose digits to binary float rounding
+                # (issue #496: '1234567890123456.78' became '...456.8').
+                # Parse plain decimal strings exactly, as the Decimal input
+                # path already does; anything else (exponents, trailing
+                # dots) keeps the previous float behaviour.
+                parts = value.lstrip("+-").split(".")
+                digits = "0123456789"
+                if (
+                    ndigits is None
+                    and len(parts) == 2
+                    and all(c in digits for c in parts[0])
+                    and parts[0]
+                    and all(c in digits for c in parts[1])
+                    and parts[1]
+                ):
+                    from decimal import Decimal
+
+                    value = Decimal(value)
+                else:
+                    value = float(value)
             else:
                 value = int(value)
         elif not isinstance(value, int):
diff --git a/tests/test_number.py b/tests/test_number.py
index 5fb12fa6..1004e106 100644
--- a/tests/test_number.py
+++ b/tests/test_number.py
@@ -53,6 +53,11 @@ def test_ordinal(test_input: str, expected: str) -> None:
         (["10311"], "10,311"),
         (["1000000"], "1,000,000"),
         (["1234567.1234567"], "1,234,567.1234567"),
+        # Regression for issue #496: grouping a large decimal string must
+        # not lose digits to binary float rounding.
+        (["1234567890123456.78"], "1,234,567,890,123,456.78"),
+        (["-1234567890123456.78"], "-1,234,567,890,123,456.78"),
+        (["14308.40"], "14,308.40"),
         (["1234567.1234567", 0], "1,234,567"),
         (["1234567.1234567", 1], "1,234,567.1"),
         (["1234567.1234567", 10], "1,234,567.1234567000"),