The fort traces its history to the expedition of Esteban Rodriguez de Figueroa in 1595. For the projected conquest of Mindanao and ultimately Brunei, a garrison and way station was established in Iloilo, the capital of Ogtong province, the name given to the southern part of Panay Island. A stone fort, dedicated initially to the N.S. del Rosario, was begun in 1616 as defense against the Dutch who were attempting to expand their hold on insular Southeast Asia. (Other sources indicate that the fort was completed in 1616 and begun in 1603.)
The fort was approximately 60 x 60 meters; its wall was 4 meters tall and 10 meters thick The Valdes Tamon report records a quadrilateral fortification with four corner bastions, three with orillons, and the fourth plain. G.J. Younghusband writing in 1899 describes the fort as “square, measuring about 80 yards each way.”
In 1617, the Dutch attacked Iloilo and in the foray burnt the fort but were repulsed by reinforcements that arrived from Manila. The fort was apparently renovated before 1738, by then it was known as San Pedro. The fort was slightly renovated in 1820 under the direction of the Maestro del Ramo de Fortificaciones, Joaquin Pabalán.
By the 19th century, Iloilo rose in prominence as an international port, which was used for the transport of raw sugar produced in the large-scale agricultural complexes in Panay and Negros. The British consul Nicolas Looney, who introduced steam-driven sugarcane presses, is credited with modernizing the extraction of sugar with the consequence greater sugar production, and hence the viability of trading sugar abroad.
While Fort San Pedro remained standing, its importance as a way station and rest area for ships and soldiers destined south waned. Spain had opened other ports elsewhere, for instance at Sual and San Esteban in Luzon, Cebu and Zamboanga in the south and shipping by the 19th century had greatly improved when steam driven metal ships were brought from Europe.
The fort was built at the mouth of the Panay River and very close to the sea, which threatened its stability. In fact, in the 1738 plan in Valdes Tamon, two bastions and a curtain facing southwest are built over the bay waters.
In February 1899, the United States took control of the fort and from 1900 to 1941 used it as a garrison for the Philippine Constabulary and the Armed forces. But as Younghusband describes the fort was in dilapidated condition such that that “its sea walls are so undermined by the action of the waves that one well-placed modern shell would tumble the whole structure into the sea.” Furthermore he adds: “Beyond the useful but hardly aggressive sandbag there are no engines of warfare in the fort, no guns in position of even the smallest calibre or most ancient pattern.” In short, the fort was useless. In 1910, an intense earthquake damaged many structures in Iloilo Province. Damage was recorded in the city and as far west as Oton. It is presumed that the fort was damaged during the earthquake.
During World War II, the Japanese used the fort a prison for war prisoners and captured USAFE forces. Much of the fortification was still standing before World War II, although dilapidated and despite threats to its stability, but in March 1945, the fort and much of the city was bombarded by American soldiers.
Damaged during the war, the ruins were demolished and the ground leveled for a park, presently called San Pedro Park. Foundations of the southwestern wall, consisting of large coral blocks, remain and are incorporated into the breakwater. The outline of two southern bastions and a stretch of curtain wall are still visible in the configuration of the breakwater.