The History of the Britons (Latin: Historia Brittonum) is a purported history of early Britain written around 828 that survives in numerous recensions from after the 11th century. The Historia Brittonum is commonly attributed to Nennius, as some recensions have a preface written in that name. Some experts have dismissed the Nennian preface as a late forgery and argued that the work was actually an anonymous compilation.

At least 5 manuscripts preserve the Historia, with the Harleian being the most complete. The Chartres manuscript was the oldest until it was largely destroyed during the accidental bombing of the library in 1944.

The Historia Brittonum describes the supposed settlement of Britain by Trojan settlers and says that Britain was named for Brutus, a descendant of Aeneas. It was the "single most important source used by Geoffrey of Monmouth in his pseudohistorical Historia Regum Britanniae" and, through the enormous popularity of this latter work, the Historia Brittonum version of Britain's early history, including the Trojan origin tradition, was incorporated into subsequent chronicles of the long-running history of the land, such as the Middle English Brut of England, also known as The Chronicles of England.

The Historia Brittonum can be dated to about 829. The work was written no earlier than the "fourth year of [the reign of] king Mermenus" (who has been identified as Merfyn Frych ap Gwriad, king of Gwynedd). Historians have conservatively assigned 828 to the earliest date for the work, which is consistent with the statement in chapter 4 that "from the Passion of Christ 796 years have passed. But from his Incarnation are 831 years".

The 19th-century classicist Theodor Mommsen divided the work into seven parts:

The work was the first source to portray King Arthur, who is described as a dux bellorum ('military leader') or miles ('warrior, soldier') and not as a king. It names the twelve battles that Arthur fought, but, unlike the Annales Cambriae, does not give any actual dates.

The reference in the Historia Brittonum to Arthur carrying the image of St. Mary on his shoulders in a battle has been interpreted by later commentators as a mistranslation of Arthur bearing the image of Mary on his shield; the words in Welsh are very similar.

The text makes use of two narrative techniques that are generally considered not reliable by modern academic standards: synthesizing and synchronizing history. Synthetic history combines legendary elements with fact, which makes the veracity of the text challenging to evaluate. Various specious causal connections and attempts to synchronize material from different sources and traditions also contribute to undermining the reliability of the chronicle.