The Corsairs of Umbar were a fleet of Men of Umbar in J. R. R. Tolkien's Middle-earth legendarium, allied to Sauron in his war against Gondor.
Umbar was an old Númenórean haven settled by the King's Men — a proud faction loyal to the King and opposed to the divine authority of the Valar. After the Downfall of Númenor, the inhabitants of Umbar took to pillaging and piracy along the coasts of Gondor. Gondor conquered Umbar in 1050 T.A., and for several centuries Umbar was a stronghold of Gondor.
At the end of the Kin-strife, the defeated rebels of Gondor (led by the Castamiri, or descendants of Castamir) fled to Umbar and usurped power there. They were bitter enemies of the legitimate kings of Gondor and again pursued piracy against Gondor. They became known as the Corsairs of Umbar. They had taken with them a large part of Gondor's fleet, thus weakening Gondor and strengthening the naval traditions of Umbar.
In 1810, King Telumehtar of Gondor conquered Umbar, killing the last of Castamir's descendants; but Umbar was lost to the Haradrim sometime later. (Tolkien gives no date for the latter event; the MERP module "UMBAR" places it in 1846.) Pirates from Umbar, still known as Corsairs, again began to harass Gondor.
By the time of the War of the Ring, the population of Umbar, mixed with the Haradrim, had lost most of its Númenórean blood, and had fallen under the domination of Sauron. Just before the Battle of the Pelennor Fields, a combined fleet amounting to "50 great ships [of Umbar] and smaller vessels beyond count" raided the port city of Pelargir in Lebennin, but these were captured by Aragorn with the aid of the Army of the Dead. Aragorn sailed the fleet to Minas Tirith to relieve the siege of the city.