Piauí passed the fortifications at Humaitá in July 1868 and provided fire support for the army for the rest of the war.

The Pará-class monitors were designed to meet the need of the Brazilian Navy for small, shallow-draft armored ships capable of withstanding heavy fire.

The monitor configuration was chosen since a turreted design did not have the same problems engaging enemy ships and fortifications as did the central battery ironclads already in Brazilian service.

The engines produced a total of 180 indicated horsepower (130 kW) which gave the monitors a maximum speed of 8 knots (15 km/h; 9.2 mph) in calm waters.

[5] Most unusually the gun's Brazilian-designed iron carriage was designed to pivot vertically at the muzzle; this was done to minimize the size of the gunport through which splinters and shells could enter.

Together with the ironclads Silvado and Cabral, Piauí passed the weakened Paraguayan fortifications at Humaitá on 21 July 1868.

Piauí, together with her sister ships Ceará and Santa Catharina, broke through the Paraguayan defenses at Guaraio on 29 April 1869 and drove off the defenders.

[8] On 31 August 1869 the monitor unsuccessfully tried to locate and destroy the remnants of the Paraguayan Navy on the Manduvirá River.