Sub-Roman Britain is the period of late antiquity in Great Britain between the end of Roman rule and the Anglo-Saxon settlement. The term was originally used to describe archaeological remains found in 5th- and 6th-century AD sites that hinted at the decay of locally made wares from a previous higher standard under the Roman Empire. It is now used to describe the period that commenced with the recall of Roman troops to Gaul by Constantine III in 407 and to have concluded with the Battle of Deorham in 577.

The end of Roman rule in Britain was the transition from Roman Britain to post-Roman Britain. Roman rule ended in different parts of Britain at different times, and under different circumstances. In 383, the usurper Magnus Maximus withdrew troops from northern and western Britain, probably leaving local warlords in charge. In 407, usurper Constantine III took the remaining mobile Roman soldiers to Gaul in response to the crossing of the Rhine in late 406, leaving the island a victim of barbarian attacks. Around 410, the Romano-British expelled the Roman magistrates from Britain. Roman Emperor Honorius replied to a request for assistance with the Rescript of Honorius, telling the Roman cities to see to their own defence, a tacit acceptance of temporary British self-government. Honorius was fighting a large-scale war in Italy against the Visigoths under their leader Alaric, with Rome itself under siege. No forces could be spared to protect distant Britain. Though it is likely that Honorius expected to regain control over the provinces soon, by the mid-6th century Procopius recognised that Roman control of Britannia was entirely lost.