Piaui was first colonised inland and slowly expanded towards the ocean. The capital, Teresina, is the only state capital in the northeast to be located inland.

The state lies almost entirely within the basin of the Parnaíba and its tributaries. Much of the state is low and swampy, as a result there are no underground sites at all. In the southeast of the state, the National Park of Serra da Capivara is a UNESCO World Heritage Site. The park has more than 400 archaeological sites and the largest concentration of rock paintings in the world. The landscape is dominated by canyons and caatinga.