Wachigabau Lake is a freshwater body crossed by the Opawica River in the southeastern part of Eeyou Istchee James Bay (municipality), in Jamésie, in the administrative region of Nord-du-Québec, in the province of Quebec, in Canada.

The Wachigabau Lake is partly formed by an enlargement of the Opawica River because of the dam at its mouth.

The surface of Wachigabau Lake is usually frozen from early November to mid-May, however, safe ice circulation is generally from mid-November to mid-April.

Formed by an enlargement of the Opawica River, this lake has a length of 31.5 kilometres (19.6 mi) shaped irregular crescent, a width maximum of 4.3 kilometres (2.7 mi) and an altitude of 302 metres (991 ft).

Tush Bay is an extension of 10.7 kilometres (6.6 mi) from the lake to East from the second outfall.