Britannia () is the national personification of Britain as a helmeted female warrior holding a trident and shield. An image first used in classical antiquity, the Latin Britannia was the name variously applied to the British Isles, Great Britain, and the Roman province of Britain during the Roman Empire. Typically depicted reclining or seated with spear and shield since appearing thus on Roman coins of the 2nd century AD, the classical national allegory was revived in the early modern period. On coins of the pound sterling issued by Charles II of England, Scotland, and Ireland, Britannia appears with her shield bearing the Union Flag. To symbolise the Royal Navy's victories, Britannia's spear became the characteristic trident in 1797, and a helmet was added to the coinage in 1825.

By the 1st century BC, Britannia replaced Albion as the prevalent Latin name for the island of Great Britain. After the Roman conquest in 43 AD, Britannia also came to refer to the Roman province that encompassed the southern two-thirds of the island (see Roman Britain). The remaining third of the island, known to the Romans as Caledonia, lay north of the River Forth in modern Scotland. It was intermittently but not permanently occupied by the Roman army. The name is a Latinisation of the native Brittonic word for Great Britain, Pretanī, which also produced the Greek form Prettanike or Brettaniai.

In the 2nd century, Roman Britannia came to be personified as a goddess, armed with a spear and shield and wearing a Corinthian helmet. When Roman Britain was divided into four provinces in 197 AD, two were called Britannia Superior (lit. 'Upper Britain') in the south and Britannia Inferior (lit. 'Lower Britain') to the north. The name Britannia long survived the end of Roman rule in Britain in the 5th century and yielded the name for the island in most European and various other languages, including the English Britain and the modern Welsh Prydain. In the 9th century the associated terms Bretwalda and Brytenwealda were applied to some Anglo-Saxon kings to assert a wider hegemony in Britain and hyperbolic inscriptions on coins and titles in charters often included the equivalent title rex Britanniae. However when England was unified the title used was rex Angulsaxonum ('king of the Anglo-Saxons').

After centuries of declining use, the Latin form was revived during the English Renaissance as a rhetorical evocation of a British national identity. Especially following the Acts of Union in 1707, which joined the Kingdoms of England and Scotland, the personification of the martial Britannia was used as an emblem of British maritime power and unity, most notably in the patriotic song "Rule, Britannia!".

A British cultural icon, she was featured on all modern British coinage series until the redesign in 2008, and still appears annually on the gold and silver "Britannia" bullion coin series. In 2015 a new definitive £2 coin was issued, with a new image of Britannia. She is also depicted in the Brit Awards statuette, the British Phonographic Industry's annual music awards.