The capital and largest city is Nueva Loja (normally referred to as Lago Agrio).

Sucumbíos is bounded on the north by Colombia, on the south by Napo and Orellana, on the west by Carchi and Imbabura, on the southwest by Pichincha, and on the east by Peru.

The eastern portion of the province is part of the Amazon Basin, with high temperatures.

Sucumbíos was an unexplored area, where only indigenous people lived, until oil was discovered in its soil.

The following table lists each with its population at the 2001 census, its area in square kilometres (km2), and the name of the canton seat or capital.