The British Iron Age is a conventional name used in the archaeology of Great Britain, referring to the prehistoric and protohistoric phases of the Iron Age culture of the main island and the smaller islands, typically excluding prehistoric Ireland, which had an independent Iron Age culture of its own. The parallel phase of Irish archaeology is termed the Irish Iron Age. The Iron Age is not an archaeological horizon of common artefacts but is rather a locally-diverse cultural phase.

The British Iron Age followed the British Bronze Age and lasted in theory from the first significant use of iron for tools and weapons in Britain to the Romanisation of the southern half of the island. The Romanised culture is termed Roman Britain and is considered to supplant the British Iron Age.

The tribes living in Britain during this time are often popularly considered to be part of a broadly-Celtic culture, but in recent years, that has been disputed. At a minimum, "Celtic" is a linguistic term without an implication of a lasting cultural unity connecting Gaul with the British Isles throughout the Iron Age. The Brittonic languages, which were widely spoken in Britain at this time (as well as others including the Goidelic and Gaulish languages of neighbouring Ireland and Gaul, respectively), certainly belong to the group known as Celtic languages. However, it cannot be assumed that particular cultural features found in one Celtic-speaking culture can be extrapolated to the others.