diff --git a/src/humanize/number.py b/src/humanize/number.py
index 52a5356a..99d3a4d1 100644
--- a/src/humanize/number.py
+++ b/src/humanize/number.py
@@ -145,6 +145,8 @@ def intcomma(value: NumberOrString, ndigits: int | None = None) -> str:
 
     For example, 3000 becomes "3,000" and 45000 becomes "45,000". To maintain some
     compatibility with Django's `intcomma`, this function also accepts floats.
+    Decimal strings without exponent notation retain their precision when
+    `ndigits` is not specified.
 
     Examples:
         ```pycon
@@ -176,6 +178,7 @@ def intcomma(value: NumberOrString, ndigits: int | None = None) -> str:
     """
     import math
 
+    decimal_value = None
     thousands_sep = thousands_separator()
     decimal_sep = decimal_separator()
     try:
@@ -184,7 +187,12 @@ 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)
+                if ndigits is None and "e" not in value.lower():
+                    from decimal import Decimal
+
+                    decimal_value = Decimal(value)
+                else:
+                    value = float(value)
             else:
                 value = int(value)
         elif not isinstance(value, int):
@@ -196,6 +204,8 @@ def intcomma(value: NumberOrString, ndigits: int | None = None) -> str:
 
     if ndigits is not None:
         result = f"{value:,.{ndigits}f}"
+    elif decimal_value is not None:
+        result = f"{decimal_value:,f}"
     else:
         result = f"{value:,}"
     if thousands_sep != "," or decimal_sep != ".":
diff --git a/tests/test_i18n.py b/tests/test_i18n.py
index 0d3ced84..5754a53a 100644
--- a/tests/test_i18n.py
+++ b/tests/test_i18n.py
@@ -140,11 +140,19 @@ def test_intcomma() -> None:
         assert humanize.intcomma("1234567,89") == "1.234.567,89"
         assert humanize.intcomma("1.234.567,89") == "1.234.567,89"
         assert humanize.intcomma("1.234.567,8") == "1.234.567,8"
+        assert (
+            humanize.intcomma("1.234.567.890.123.456,78") == "1.234.567.890.123.456,78"
+        )
+        assert humanize.intcomma("-1234567890123456,78") == "-1.234.567.890.123.456,78"
+        assert humanize.intcomma("1234,1200") == "1.234,1200"
 
         humanize.i18n.activate("fr_FR")
         assert humanize.intcomma(number) == "10 000 000"
         assert humanize.intcomma(1_234_567.89) == "1 234 567,89"
         assert humanize.intcomma("1 234 567,89") == "1 234 567,89"
+        assert (
+            humanize.intcomma("1 234 567 890 123 456,78") == "1 234 567 890 123 456,78"
+        )
 
         humanize.i18n.activate("pt_BR")
         assert humanize.intcomma(number) == "10.000.000"
diff --git a/tests/test_number.py b/tests/test_number.py
index 5fb12fa6..88af55cf 100644
--- a/tests/test_number.py
+++ b/tests/test_number.py
@@ -83,6 +83,41 @@ def test_intcomma(
     assert humanize.intcomma(*test_args) == expected
 
 
+@pytest.mark.parametrize(
+    "value, expected",
+    [
+        ("1234567890123456.78", "1,234,567,890,123,456.78"),
+        ("-1234567890123456.78", "-1,234,567,890,123,456.78"),
+        ("0.1234567890123456789", "0.1234567890123456789"),
+        ("0.000000000000000000123456789", "0.000000000000000000123456789"),
+        ("1234.1200", "1,234.1200"),
+        ("+001234.1200", "1,234.1200"),
+    ],
+)
+def test_intcomma_decimal_string_precision(value: str, expected: str) -> None:
+    assert humanize.intcomma(value) == expected
+
+
+@pytest.mark.parametrize(
+    "value, ndigits, expected",
+    [
+        ("1234.545", 2, "1,234.55"),
+        ("1234.1200", 3, "1,234.120"),
+        ("1.0e3", None, "1,000.0"),
+        ("1e3", None, "1e3"),
+        ("bad.12", None, "bad.12"),
+        ("1.2.3", None, "1.2.3"),
+        ("nan", None, "NaN"),
+        ("-inf", None, "-Inf"),
+        ("1.0e400", None, "+Inf"),
+    ],
+)
+def test_intcomma_decimal_string_controls(
+    value: str, ndigits: int | None, expected: str
+) -> None:
+    assert humanize.intcomma(value, ndigits) == expected
+
+
 @pytest.mark.parametrize("sign", [1, -1])
 def test_intcomma_large_integer(sign: int) -> None:
     value = sign * (10**400 + 123)