diff --git a/mssql_python/cursor.py b/mssql_python/cursor.py
index 05324875..537ca046 100644
--- a/mssql_python/cursor.py
+++ b/mssql_python/cursor.py
@@ -2322,6 +2322,20 @@ def executemany(  # pylint: disable=too-many-locals,too-many-branches,too-many-s
                     paraminfo.paramSQLType = ddbc_sql_const.SQL_VARCHAR.value
                     paraminfo.columnSize = 1
 
+                # Special handling for Decimal columns sent as SQL_VARCHAR (GH-557)
+                # The column_size was computed from a single sample value, but
+                # negative signs can make other rows' formatted strings longer.
+                # Scan all rows to find the true maximum formatted length.
+                if paraminfo.paramSQLType == ddbc_sql_const.SQL_VARCHAR.value:
+                    max_decimal_size = paraminfo.columnSize
+                    for row in seq_of_parameters:
+                        value = row[col_index]
+                        if value is not None and isinstance(value, decimal.Decimal):
+                            formatted_len = len(format(value, "f"))
+                            if formatted_len > max_decimal_size:
+                                max_decimal_size = formatted_len
+                    paraminfo.columnSize = max_decimal_size
+
                 # Special handling for binary data in auto-detected types
                 if paraminfo.paramSQLType in (
                     ddbc_sql_const.SQL_BINARY.value,
diff --git a/tests/test_004_cursor.py b/tests/test_004_cursor.py
index 17e06961..214964e6 100644
--- a/tests/test_004_cursor.py
+++ b/tests/test_004_cursor.py
@@ -2304,6 +2304,54 @@ def test_executemany_Decimal_list(cursor, db_connection):
         db_connection.commit()
 
 
+def test_executemany_decimal_sign_change(cursor, db_connection):
+    """Test executemany with decimals that change signs (GH-557).
+
+    When the sample value chosen for column sizing is shorter than a negative
+    value in the batch, the formatted string (with a leading '-') can exceed
+    the allocated column_size, causing a RuntimeError.
+    """
+    try:
+        cursor.execute("CREATE TABLE #pytest_decimal_sign (col_1 DECIMAL(28, 14))")
+
+        # Case 1: negative first, then positive — previously worked
+        data1 = [(decimal.Decimal("-0.1"),), (decimal.Decimal("1.0"),)]
+        cursor.executemany("INSERT INTO #pytest_decimal_sign VALUES (?)", data1)
+
+        # Case 2: positive first, then negative — previously failed
+        data2 = [(decimal.Decimal("0.1"),), (decimal.Decimal("-0.1"),)]
+        cursor.executemany("INSERT INTO #pytest_decimal_sign VALUES (?)", data2)
+
+        # Case 3: positive then negative with different integer parts
+        data3 = [(decimal.Decimal("1.0"),), (decimal.Decimal("-0.1"),)]
+        cursor.executemany("INSERT INTO #pytest_decimal_sign VALUES (?)", data3)
+
+        # Case 4: multiple sign changes in a single batch
+        data4 = [
+            (decimal.Decimal("100.5"),),
+            (decimal.Decimal("-0.001"),),
+            (decimal.Decimal("0.5"),),
+            (decimal.Decimal("-999.99"),),
+        ]
+        cursor.executemany("INSERT INTO #pytest_decimal_sign VALUES (?)", data4)
+
+        db_connection.commit()
+
+        # Verify row count
+        cursor.execute("SELECT COUNT(*) FROM #pytest_decimal_sign")
+        count = cursor.fetchone()[0]
+        assert count == 10
+
+        # Verify data correctness for the originally-failing case
+        cursor.execute("SELECT col_1 FROM #pytest_decimal_sign ORDER BY col_1")
+        rows = [row[0] for row in cursor.fetchall()]
+        assert decimal.Decimal("-999.99") in [r.quantize(decimal.Decimal("0.01")) for r in rows]
+        assert decimal.Decimal("0.1") in [r.quantize(decimal.Decimal("0.1")) for r in rows]
+    finally:
+        cursor.execute("DROP TABLE IF EXISTS #pytest_decimal_sign")
+        db_connection.commit()
+
+
 def test_executemany_DecimalString_list(cursor, db_connection):
     """Test executemany with an string of decimal parameter list."""
     try: