Longlac is a community in the municipality of Greenstone, in northwestern Ontario, Canada. It is located along Highway 11 and the Canadian National Railway, on the namesake Longlac Bay at the northern end of Long Lake.

Greenstone is a single-tier municipality in the Canadian province of Ontario with a population of 4,636 according to the 2016 Canadian census. It stretches along Highway 11 from Lake Nipigon to Longlac and covers 2,767.19 km2 (1,068.42 sq mi).