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. 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, the Roman-ruled portion of the island 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 late 3rd century, 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 296, the Diocese of the Britains (with its vicar at Londinium) was formed and later subordinated to the praetorian prefecture of Gaul.
According to the Verona List and the Notitia Dignitatum, 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.
The placement and capitals of these late British provinces are uncertain, although the Notitia Dignitatum lists the governor (praeses) of Britannia II as being of equestrian rank, making him unlikely to have been based in Londinium. The list of bishops who attended the 314 Council of Arles is patently corrupt but generally assumed to have mimicked the Roman administration: it seems certain one of the bishops was from Eboracum, even if his name ("Eborius") was a scribal error.
Ammianus records that in the year 369 Count Theodosius established or refounded the province of Valentia (further attested in the Notitia Dignitatum) from lands recaptured from "the enemy". Its location is a matter of scholarly debate, but some place it at Hadrian's Wall in the area around Luguvalium (Carlisle). If so, it would have been formed at some point in the 4th century out of territory formerly administered from Eboracum. Others place it between Hadrian's Wall and the Antonine Wall or in Wales in the area around Deva (Chester); in both cases, some of the territory also may have been formerly administered from Eboracum prior to the reorganization.
- Frere, pp. 198-199.
- Polemius Silvius's 5th-century Nomina Omnium Provinciarum gives six provinces, but Roman administration over the Orcades (Orkneys) is generally discounted.
- Dornier, Ann (1982). "The Province of Valentia". Britannia 13: 253–260. doi:10.2307/526498.
- Valentia is generally treated as a later formation and placed variously beyond the Wall, around the Wall, and in Wales. It may, however, have simply been another name for the British diocese as a whole.
- Labbé, Philippe & Gabriel Cossart (eds.) Sacrosancta Concilia ad Regiam Editionem Exacta: quae Nunc Quarta Parte Prodit Actior [The Sancrosanct Councils Exacted for the Royal Edition: which the Editors Now Produce in Four Parts], Vol. I: "Ab Initiis Æræ Christianæ ad Annum CCCXXIV" ["From the Beginning of the Christian Era to the Year 324"], col. 1429. The Typographical Society for Ecclesiastical Books (Paris), 1671.
- Thackery, Francis. Researches into the Ecclesiastical and Political State of Ancient Britain under the Roman Emperors: with Observations upon the Principal Events and Characters Connected with the Christian Religion, during the First Five Centuries, pp. 272 ff. T. Cadell (London), 1843.
- "Nomina Episcoporum, cum Clericis Suis, Quinam, et ex Quibus Provinciis, ad Arelatensem Synodum Convenerint" ["The Names of the Bishops with Their Clerics who Came Together at the Synod of Arles and from which Province They Came"] from the Consilia in Thackery (Latin)
- Ammianus, XXVIII, iii.
- Frere, Sheppard (1967). Britannia: a history of Roman Britain. Cambridge: Harvard University Press.
- Mattingly, David (2006). An Imperial Possession: Britain in the Roman Province. London: Penguin.
- Creighton, John (2006). Britannia: the Creation of a Roman Province. London and New York: Routledge.