Thénia (الثنية), sometimes written as Thenia, with around 40,000 inhabitants, is the chief town in the daïra of the same name, in the wilaya of Boumerdès, in northern Algeria.

Historically, the name is a contraction of "Theniet Beni Aicha" (ثنية بني عائشة) ("the mountain pass of the sons of Aisha"), the Arabic translation of the Kabyle Berber toponym Tizi n At Ɛica.

The steep-sided pass, which is only about 800 metres (2,600 ft) wide at its narrowest point, is sometimes taken to mark the transition between Mitidja and Grande Kabylie.

South and east is the valley of the Isser River, whose sides rise to around 600 metres (2,000 ft) and are deeply incised by streams.

Thénia is on the double-track portion of the Algiers-Skikda railway line and is the end of electric commuter rail service from Algiers station.