Allectus (died 296) was a Roman emperor in Britain (293–296).

Allectus was the treasurer of Carausius. When Carausius lost control of northern Gaul and its crucial port of Boulogne, Allectus assassinated him and took control, probably in autumn 293.

His reign has left little record, although his coin issues display a similar distribution to those of Carausius. They are found in north western Gaul, indicating that the recapture of Boulogne did not spell the end of the rebel empire on that side of the English Channel.

However, Allectus proved unable to prevent the invasion of Constantius I, who launched a two-pronged attack on Britain in September 296. Although at least part of the first invasion force under Constantius seems to have been turned back by storms, the second force under the praetorian prefect Asclepiodotus successfully landed near Southampton Water, where he burnt his boats and marched toward London. Allectus confronted him near Silchester, but lacking his predecessor's strategic skill was defeated and killed in battle.

Constantius himself arrived to protect London from Allectus' retreating Frankish troops and receive a triumphant victory.

Geoffrey of Monmouth included Allectus in his semi-mythical mediaeval history and wrote that as soon as Allectus assumed the kingship of Britain, he massacred hundreds of supporters of Carausius for breaking their allegiance to Rome.