Flavia Caesariensis was one of the provinces of Roman Britain.

It was created in the early 4th century AD under the reforms of Diocletian and had its capital at Lincoln. Its boundaries are uncertain but probably consisted of the southern Pennines, stretching west to the Irish Sea and may have extended far enough south to encompass the territory of the Iceni. The capital of Britannia Secunda at York indicates that Flavia's territory did not extend much further north of Lincoln. It is referred to in the Verona List which dates to 312 -314.

Flavia Caesariensis and its southern neighbour, Maxima Caesariensis, may have briefly comprised a single province which covered most of what is now eastern England. Eric Birley and others have suggested that the two provinces named Caesariensis were descended from a larger unit which received its unusual name due to Imperial favour. After London welcomed Constantius in 296 he argues that he may have granted it the additional title of 'Caesariensis'. Given the circumstances at the time Constantius may well have divided the military command of Britannia Superior in two with an eastern province named Britannia Caesariensis and a western one named Britannia Prima. This division would have been later reorganised by splitting Britannia Caesariensis into north and south regions named after Constantius and the western Augustus, Marcus Aurelius Valerius Maximianus This would require the split to have taken place prior to Maximian's abdication in 305. As Constantius was elevated to the title of Augustus in the same year, London may also have adopted the title at the same time, as indeed it was later known. The short period of it holding the title Caesariensis may explain why there is little other evidence of that name.

An alternative explanation is that the province was named after Flavius Constantius, emperor of the west and that Maxima Caesarensis was named in honour of Galarius Maximianus , emperor of the east. This raises the question of why an eastern emperor was honoured and the two Augusti of the Tetrachy were not.