That thin strip of coast is where the Parnaíba River finally arrives. The Parnaíba runs 1,400 kilometres and forms the entire western boundary of the state, separating Piauí from Maranhão, and it is the organising fact of Piauí's geography the way the São Francisco is for Bahia. At its mouth it breaks into a delta of mangrove, dune, beach and marsh, protected since 1996 by an Environmental Protection Area of 313,800 hectares. The river gave the state its access to the world before roads did, and the name Piauí itself is traced to the piaba, a small river fish. A state named for a minnow, with almost no ocean frontage, that nevertheless built its identity on water.

Francisco Dias d'Ávila came from Bahia; Domingos Afonso Mafrense came from Portugal and founded what is now Oeiras, which would serve as the capital of the Piauí captaincy until 1852. Between them, from 1674 onwards, they laid down the pattern that would define the interior for two centuries - vast cattle ranches on open savanna, small populations, long distances, and an economy that moved on the hoof. The war of independence still reached it. On 13 March 1823, at the Jenipapo River, Piauian irregulars fought Portuguese troops in one of the engagements of Brazil's war of independence. It matters enough to the state that in 2005 the date was added in words to the state flag, below the single star, where it remains: 13 DE MARÇO DE 1823.

In 1852 an architect sat down and designed a city. Oeiras had become impossible to run a province from - too far from anywhere, too hard to trade through - so on 16 August 1852 the capital was refounded on the Parnaíba as Vila Nova do Poty, named for the second river that joins the first at the city's northern edge. Teresina was the first planned city in Brazil. It remains the only state capital in the Northeast that does not sit on the coast, which follows logically from everything else about Piauí. Today the state has 224 municipalities and a population of 3,271,199 by the 2022 census - a density of thirteen people per square kilometre, which in practice means long empty highways and towns that appear without warning. The 2022 census recorded 64.8 percent of Piauians as brown or multiracial, 22.6 percent white and 12.2 percent Black. By the IBGE's measure this is the most Catholic state in Brazil and the least Protestant.

Three landscapes meet inside Piauí. Caatinga thorn scrub covers the southeast, Cerrado savanna the south and west, and along the northern lowlands stand the babaçu palm forests, the mata dos cocais, where the climate turns hot and humid. In the uplands the days are fierce and the nights turn cool. Set into this are two national parks of international importance. Sete Cidades, created on 8 June 1961 and covering 7,700 hectares in the municipalities of Brasileira and Piracuruca, is a scatter of weathered rock monuments carrying prehistoric paintings and inscriptions. Far to the southwest, Serra da Capivara - 1,291 square kilometres, a UNESCO World Heritage Site since 1991 - holds what UNESCO recognises as the largest and oldest concentration of prehistoric sites in the Americas, and the dates coming out of it have kept the question of when people first reached this hemisphere unsettled for decades. Both sit in a state most Brazilians have never visited.

Piauí is one of the poorest states in Brazil. Its human development index stood at 0.764 in 2024, nineteenth among the states, and transfers from the federal government are estimated to make up around a third of state GDP. Services account for roughly 60 percent of that GDP, industry 27 percent and agriculture 13 percent; exports have run to essential oils, soybeans, woven cotton, cashews, crustaceans and leather. The largest industrial company headquartered here, Grupo Claudino, turns over around 700 million reais a year. The recent story is agricultural expansion into the Cerrado: substantial tax incentives brought Bunge, Cargill and Algar in to grow soy, and Suzano to grow eucalyptus for pulp. State environmental activists have criticised those projects, and the argument over what the southern savanna is for has not been settled.