The Battle of Diamond Rock took place between 31 May and 2 June 1805 during the Napoleonic Wars, when a Franco-Spanish force dispatched under Captain Julien Cosmao was able to retake Diamond Rock, on the approach to Fort-de-France, from the British forces that had occupied it over a year before.

The French in Martinique had been unable to oust the defenders from the strategically important rock, allowing the British garrison to restrict access to Fort-de-France Bay, firing on ships attempting to enter it by the quickest route with guns they had placed on the cliffs and thus forcing them to take a slower route, allowing interception by the blockading vessels. The arrival of a large combined Franco-Spanish fleet in May changed the strategic situation. The French commander, Pierre de Villeneuve, had vague orders to attack British possessions in the Caribbean, but instead waited at Martinique for clearer instructions. He was finally persuaded to authorise an assault on the British position, and a Franco-Spanish flotilla was dispatched to storm the rock. Already short of water, the defenders held on in the summit for several days, while the French, who had neglected to bring scaling ladders, could make little headway.

The British, short of both water and ammunition, eventually negotiated the surrender of the rock after several days under fire. As Diamond Rock was legally considered a Royal Navy vessel, and the commander was legally "captain" of it, after repatriation, he was tried by court-martial (as the law dictated in any case where a captain loses his ship, regardless of the cause), but was honourably acquitted.

Diamond Rock had been fortified in January 1804 on the orders of Commodore Samuel Hood. Hood had been active in the West Indies, protecting British convoys from French privateers issuing out of the two major naval bases the French retained in the Caribbean, at Guadeloupe and Martinique. The privateers had captured a number of valuable cargoes and were diverting British warships to protect the merchant fleets. Hood decided to blockade Martinique, and thus curtail the privateers and intercept supplies destined for the French garrison. Patrolling off the southern end of the island, Hood saw that if Diamond Rock could be occupied, it would allow the British to effectively control the quickest shipping approaches to Fort-de-France & St. Pierre on the western side, as the strong easterly currents & prevailing easterly winds around the island made the easiest approaches mean passing within sight of Diamond Rock.

Hood reconnoitered Diamond Rock and considered it excellently defensible, with the only possible landing site being on the western side. He wrote that "thirty riflemen will keep the hill against ten thousand ... it is a perfect naval post." A party of men were landed on 7 January 1804, from Hood's flagship HMS Centaur, under the command of Centaur's first lieutenant James Wilkes Maurice. They promptly fortified the small cove they had landed at with their launch's 24-pounder, and established forges and artificers' workshops in a cave at the base of the rock. After fixing ladders and ropes to scale the sheer sides of the rock, they were able to access the summit and began to establish messes and sleeping areas in a number of small caves. Bats were driven out by burning bales of hay, and a space was cleared by blasting at the top of the rock in order to establish a battery. In February a number of guns were transferred over from Centaur, with two 24-pounders being installed in a cave near sea level, another 24-pounder halfway up the rock, and two 18-pounders in the battery at the top. In addition to this the men had use of a number of boats, with one armed with a 24-pounder carronade, which were used to intercept enemy ships.

Marshall's Naval Biography, when describing the process of hauling the guns to the summit, recorded that

Lieutenant Maurice having succeeded in scrambling up the side of the rock ... and fastened one end of an 8-inch hawser to a pinnacle, the viol-block was converted into a traveller, with a purchase-block lashed thereto, and the other end of the hawser set up, as a jack-stay, round the Centaur's main-mast. The gun being slung to the viol, the purchase-fall was brought to the capstern. In this manner the desired object was effected in the course of a week, during which time Lieutenant Maurice and the working party on shore suffered most dreadfully from excessive heat and fatigue, being constantly exposed to the sun, and frequently obliged to lower themselves down over immense precipices to attend the ascent of the guns, and bear them off from the innumerable projections against which they swung whenever the ship took a shear...

Despite the vulnerability of both Centaur and the Rock to a French gunboat attack while the process of fortification was being carried out, the French neglected to act. The governor of Martinique, Louis Thomas Villaret de Joyeuse, ordered work to begin on building a road to the coast opposite the rock, and the establishment of a battery there, but the British were forewarned by the black population of the island who were largely sympathetic to the British. A party was sent onshore, which succeeded in capturing the engineer sent to construct the battery, and three of his men. Work on the battery was abandoned after further British raids on the area.