Juan Prim, Marquis of los Castillejos, Count of Reus (December 12, 1814 – December 30, 1870), Spanish soldier and statesman, was the son of Lieut.-Colonel Pablo Prim, and was born at Reus in Catalonia. He entered the free corps known as the volunteers of Isabella II in 1834, and in the course of the Carlist War he rose to the rank of lieutenant-colonel and had two orders of knighthood conferred upon him. After the pacification of 1839, as a progressist opposed to the dictatorship of Espartero, he was sent into exile. However, in 1843 he was elected deputy for Tarragona, and after defeating Espartero at Bruch he entered Madrid in triumph with Serrano . The regent Maria Christina promoted him major-general, and made him count of Reus.

Narvaez , the prime minister, failed to understand what constitutional freedom meant, and Prim, on showing signs of opposition, was sentenced to six years' imprisonment in the Philippine Islands. The sentence was not carried out, and Prim remained an exile in England and France until the amnesty of 1847. He then returned to Spain, and was first employed as captain-general of Puerto Rico (Governor of Puerto Rico) and afterwards as military representative with the sultan during the Crimean War. In 1854 he was elected to the cortes, and gave his support to O'Donnell, who promoted him lieutenant-general in 1856. In the war with Morocco he did such good service at Los Castillejos or Marabout, Cabo Negro, Guad al Gelu and Campamento in 1860 that he was made marquis de los Castillejos and a grandee of Spain.

He commanded the Spanish army in Mexico when he refused to consent to the ambitious schemes of Napoleon III. On his return to Spain he joined the opposition, heading pronunciamentos in Catalonia against Narvaez and O'Donnell. All his attempts failed until the death of Narvaez in April 1868, after which Queen Isabella fell more and more under the influence of the Jesuits, and became increasingly tyrannical, until at last even Serrano was exiled. In September 1868 Serrano and Prim returned, and Admiral Topete, commanding the fleet, raised the standard of revolt at Cádiz. In July 1869 Serrano was elected regent, and Prim became president of the council and was made a marshal. On the 6th of November 1870 Amadeo, duke of Aosta, was elected king of Spain, but Prim, on leaving the chamber of the cortes on the 28th of December, was shot by unknown assassins and died two days later. The cortes took his children as wards of the country; three days afterwards King Amadeo I. swore in the presence of the corpse to observe the new Spanish constitution.

Two biographies of Prim down to 1860 were published in that year by Gimenez y Guited and Gonzalez Llanos. See also L Blairet, Le General Prim et la situation actuelle de l'Espagne (Paris, 1867); Guillaumot, Juan Prim et l'Espagne (Paris, 1870); and Prim, by H Leonardon (in French, 1901), which contains a useful bibliography.