Newfoundland and Labrador–Quebec border
The border between the provinces of Quebec and of Newfoundland and Labrador is the longest interprovincial border in Canada. It stretches for more than 3,500 kilometres (2,200 mi) on land, and, according to both provincial governments, also contains a maritime part.
Source: Wikipedia — Newfoundland and Labrador–Quebec border (CC BY-SA 4.0)