The Bras d'Isaïe (English: arm of Isaiah) is a tributary of the rivière à Mars, flowing in the unorganized territory of Lac-Ministuk, in the Le Fjord-du-Saguenay Regional County Municipality, in the administrative region of Saguenay–Lac-Saint-Jean, in the province of Quebec, in Canada.

A few secondary forest roads serve the valley of the "Bras d'Isaïe", especially for forestry and recreational tourism activities.

The surface of the "Bras d'Isaïe" is usually frozen from the beginning of December to the end of March, however the safe circulation on the ice is generally made from mid-December to mid-March.

The main watersheds neighboring the "Bras d'Isaïe" are: The "Bras d'Isaïe" rises at the mouth of "Lac du Bec" (length: 0.2 km (0.12 mi); altitude: 358 m (1,175 ft)) which is located in the unorganized territory of Lac-Ministuk.

The toponym "Bras d'Isaïe" was formalized on June 29, 1983, at the Place Names Bank of the Commission de toponymie du Québec.