Britannia Secunda or Britannia II (Latin for "Second Britain") was one of the provinces of the Diocese of "the Britains" created during the Diocletian Reforms at the end of the 3rd century. It was probably created after the defeat of the usurper Allectus by Constantius Chlorus in AD 296 and was mentioned in the c. 312 Verona List of the Roman provinces. Its position and capital remain uncertain, although it probably lay further from Rome than Britannia I. At present, most scholars place Britannia II in Yorkshire and northern England. If so, its capital would have been Eboracum (York).
Following the Roman conquest of Britain, it was administered as a single province from Camulodunum (Colchester) and then Londinium (London) until the Severan Reforms following the revolt of its governor Clodius Albinus. These divided the territory into Upper and Lower Britain (Britannia Superior and Inferior), whose respective capitals were at Londinium and Eboracum. During the first phases of the Diocletian Reforms, Britain was under the control of the Allectus's Britannic Empire as part of the Carausian Revolt. At some point after the territory was retaken by Constantius Chlorus in AD 296, the Diocese of the Britains (with its vicar at Londinium) was formed and made a part of Prefecture of Gaul. The Britains were divided among three, four, or five provinces, which seem to have borne the names Prima, Secunda, Maxima Caesariensis, and (possibly) Flavia Caesariensis and Valentia.