Genseric (c. 389 – January 25, 477), also spelled as Geiseric or Gaiseric, was King of the Vandals and Alans (428–477) and was one of the key players in the troubles of the Western Roman Empire in the 5th century. During his nearly 50 years of rule, he raised a relatively insignificant Germanic tribe to the status of a major Mediterranean power — which, after he died, entered a swift decline and eventual collapse.

Succeeding his brother Gunderic at a time when the Vandals were settled in Baetica (modern Andalusia, Spain), Gaiseric transported all his people, around 80,000, to to Northern Africa in 428. He might have been invited by the Roman governor Bonifacius, who wished to use the military strength of the Vandals in his struggle against the imperial government.

Gaiseric caused great devastation as he moved eastward from the Strait of Gibraltar across Africa. He turned on Bonifacius, defeated his army in 430, and then crushed the joint forces of the Eastern and Western empires that had been sent against him. In 435 Gaiseric concluded a treaty with the Romans under which the Vandals retained Mauretania and part of Numidia as federates (allies under special treaty) of Rome. In a surprise move on October 19, 439, Gaiseric captured Carthage, striking a devastating blow at imperial power. In a 442 treaty with Rome the Vandals were recognized as the independent rulers of Byzacena, and part of Numidia. Gaiseric’s fleet soon came to control much of the western Mediterranean, and he annexed the Balearic Islands, Sardinia, Corsica, Malta and Sicily.