The river Ternoise (French pronunciation: [tɛʁnwaz]; Dutch: Ternaas) is one of the small chalk streams that flow from the plateau of the southern Boulonnais and Picardy, via the Canche, into the English Channel.

The basin of the Ternoise extends to 342 square kilometres (132 sq mi) and lies in the southern end of the département of Pas-de-Calais.

The 41.4-kilometre (25.7 mi) long river[1] rises at Ligny-Saint-Flochel and passes through Saint-Pol-sur-Ternoise to join the river Canche at Huby-Saint-Leu, near to the town of Hesdin.

Seasonal flow fluctuations are not very marked, similar to the Canche or the Somme, its neighbours.

The higher water flows occur at the end of winter and in the spring.