The El Niño events of 1891, 1925, 1983 and 1998 repeatedly overflowed the Piura river, flooding Castilla and Catacaos and causing the collapse of the Bolognesi bridge on March 16, 1998. To solve the floods and the water deficit, Decree Law 18381 of September 1, 1970 created the Chira-Piura Special Project: the Poechos Dam (one billion cubic meters), a 54 km transfer canal, the Sullana Diversion Dam and the Miguel Checa Canal irrigate more than 100,000 hectares and protect the cities from extraordinary floods.

After the bankruptcy caused by the war, the province of Piura reconverted itself with the acclimatization of Pima Cotton in 1918, whose extra-long staple fiber conquered the markets of Great Britain and North America, enriching the Chira and Piura valleys. Simultaneously, the La Brea and Pariñas oil fields, operated by London Pacific Petroleum and later the International Petroleum Company, made Talara the energy heart of the country, until the military takeover and nationalization of the enclave in October 1968 under Juan Velasco Alvarado.

Piura gave the country its greatest maritime hero: Admiral Miguel Grau Seminario, born on Tacna street in Piura, who commanded the monitor Huáscar until his death at Angamos on October 8, 1879. The «Piura N.° 25» and «Manco Cápac» battalions suffered heavy casualties at San Juan and Miraflores (1881). In September 1880 the expedition of Patricio Lynch bombarded Paita, imposing a contribution of 10,000 pesos and destroying the customs house, the pier and the railway workshops; between 1881 and 1883 Chilean troops occupied Piura and Catacaos.

In 1872, under the government of José Balta, construction of the Paita–Sullana–Piura Railway began under Federico Blume Othon, with a standard gauge of 1,435 m and 105 km of track. It reached La Huaca in 1874, Viviate in 1876, Sullana in 1879 and its terminal station in Piura in 1887, linking the harvests of the Chira and Piura valleys with the ships anchored at Paita. It operated until the mid-20th century under the Peruvian Corporation. In 1893 the «Old Bridge» (San Miguel) connected the city with Castilla across the Piura river.

After depending on the Prefecture of Trujillo during its first republican decades, the demographic and commercial weight of Piura demanded administrative autonomy. Through the Law of March 30, 1861, during the second government of Marshal Ramón Castilla, the Department of Piura was officially created with the founding provinces of Piura, Paita and Ayabaca.

Led by Miguel Gerónimo Seminario y Jaime and councilman Buenaventura Raygada, armed groups peacefully disarmed the royalist garrison on the night of January 3, 1821. On January 4, before a massive crowd that overflowed the town hall, the session moved to the church of the Convent of San Francisco de Asís, where the independence of Piura was solemnly declared. The proclamation counted on artisans, the communities of Catacaos and Sechura, Black militiamen and women of all social ranks.

Enlightened ideas circulated in Piura through the Mercurio Peruano, fostering identity and unrest against the tributes and monopolies of the Crown. In October 1820 Guayaquil proclaimed its independence, closing the royalist flank, while Cochrane squadron and San Martín landing at Paracas dismantled viceregal control. On December 29, 1820, Torre Tagle proclaimed the independence of Trujillo and dispatched letters to the subdelegations, including Piura.