Oust-Marest is a commune in the Somme département in Hauts-de-France in northern France.

The commune is situated on the D1015 road, some 20 miles (32 km) southwest of Abbeville, on the banks of the river Bresle, the border with Seine-Maritime.

Possibly from a settlement named in honour of Augusta.

Archaeologists have found traces of a Gallo-Roman amphitheatre and a temple in the locality.

This Arrondissement of Abbeville geographical article is a stub.