Fos-sur-Mer (French pronunciation: [fɔs syʁ mɛʁ], literally Fos on Sea; Provençal: Fòs) is a port town and commune in the Bouches-du-Rhône department in southern France. It is one of the busiest ports in Europe despite its small size.

Fos-sur-Mer is situated about 50 km (31 mi) north west of Marseille, on the Mediterranean coast, and to the west of the Étang de Berre. The city has 6 km (3.7 mi) of sand beach.

Fos is the site of a major port development operated by the Autonomous Port of Marseille. The facilities include container handling terminals and a gas (methane) terminal. The waterside location of the industrial zone is attractive to heavy industry including steel.

The steel group ArcelorMittal has its Sollac Méditerranée plant here (merged into ArcelorMittal in 2006). The presence of the steel, chemistry and oil industries means that pollution levels are high.

Fos-sur-Mer is home to Fos Provence Basket which plays its home games at the 2,000 seat Complexe sportif Parsemain.