Tortefontaine lies southwest of Hesdin, on the D136 road with the Varnette, a small tributary of the Authie flowing through it.

The border character of the Authie valley, today between the Pas-de-Calais and the Somme, dates back to the Middle Ages when it separated the kingdom of France from the Spanish Netherlands, to the north.

To the west, the river opens onto the Bay of Authie, typical of the Picardy estuary, and flows into the English Channel.

The altitude of the plateau declines from the Doullens region in the east (highest point at 163 m) to the Picardy lowlands in the west (less than 40 m).

The Authie is a coastal river classified as a first-class watercourse where the dominant fish population is made up of salmonids.