The Saulx () is a 115 km long river in France, right tributary of the Marne. Its source is near the village Germisay. It flows generally northwest, through the following départements and towns:
The Saulx flows into the Marne in Vitry-le-François. Its main tributaries are the Ornain and the Chée. The part of the Marne-Rhine Canal between Vitry-le-François and Sermaize-les-Bains runs parallel to the Saulx.