The Authie (French pronunciation: [o.ti] ⓘ; Picard: Eutie; (West Flemish: Otie)) is a river in northern France whose 108-kilometre (67 mi) course crosses the departement of the Pas-de-Calais and the Somme.

The river then goes to Doullens, where it converges with its main tributary (the Grouche [fr]), followed by Auxi-le-Château, and then Argoules and Nampont to the north of the forest of Crécy, before it flows into the English Channel between Fort-Mahon-Plage and Berck.

The Authie cuts into the wide plateau sloped towards the west that contains Artois and Ponthieu; a small layer of Pleistocene silt covers the flint clay and a base of chalk.

The profile along the river is rather consistent and characterized by a small slope of average (1%), more intense upstream of Doullens (2.3%), and not more than 0.4% downstream of Dompierre-sur-Authie making the Authie a slow flowing stream.

[9] The Authie benefits from an extremely straightforward drainage basin (of 1,305 square kilometers (504 sq mi))[7] that corresponds to a valley in Artois, where the river collects a symmetrical network of basic tributaries.

[10] This lack of symmetry is a result of the different sensitivities of the slopes to frost weathering linked to a cycle of freezing and thawing during the periods of Quaternary glaciation.

[18] If the valley was inhabited in the Gallic period (jewelry, weapons, and coins have been discovered at Dompierre-sur-Authie in a sanctuary of the pre-Roman era), or while it was part of Gallia Belgica, the course of the Authie remains away from large population movements.

The lanes of communication do not follow the valley from east to west but instead cross it north to south, including the tin road that connected what is now the port of Boulogne-sur-Mer to southern France before the period of Roman occupation[19] as well as the Roman roads securing contact between Lutetia (Paris), Samarobriva (Amiens), and the English Channel through the port of Gesoriacum (Boulogne-sur-Mer).

In 1272, Jean de Nesle, husband of Joan, Countess of Ponthieu, envisioned the digging of a canal from the bay of the Authie to Rue, whose port had begun to silt up.

[22] Due to the many combats that took place in the valley, the requisitions of the armies led to the destruction of many buildings and villages and drove the inhabitants to arrange muches [fr], or underground shelters.

[22] Boundary problems do not end with the fall of the Ancien Régime and the establishment of the departments by the French Revolution, as from 1790 to 1791, Auxi-le-Château remained divided in two, between the Somme and Pas-de-Calais before becoming part of the latter.