Historia regum Britanniae (The History of the Kings of Britain), originally called De gestis Britonum (On the Deeds of the Britons), is a fictitious account of British history, written around 1136 by Geoffrey of Monmouth. It chronicles the lives of the kings of the Britons over the course of two thousand years, beginning with the Trojans founding the British nation and continuing until the Anglo-Saxons assumed control of much of Britain around the 7th century. It is one of the central pieces of the Matter of Britain.

Although taken as historical truth until the 16th century, it is now considered to have no value as history. When events described, such as Julius Caesar's invasions of Britain, can be corroborated from contemporary histories, Geoffrey's account can be seen to be wildly inaccurate. It remains, however, a valuable piece of medieval literature, which contains the earliest known version of the story of King Lear and his three daughters, and helped popularise the legend of King Arthur.

Geoffrey starts the book with a short dedication. He claims that he was given a source for the time period by Archdeacon Walter of Oxford, who presented him with a "certain very ancient book written in the British language" from which he has translated his history.

The Historia begins with the Trojan hero Aeneas, who, according to the Aeneid, settled in Italy after the Trojan War. His great-grandson Brutus is banished, and, after a period of wandering, is directed by the goddess Diana to settle on an island in the western ocean. Brutus lands at Totnes and names the island "Britain" after himself, defeating the previous giant inhabitants. He establishes a city eventually renamed London. After his death, Brutus divides the island into three kingdoms named Loegria, Kambria and Albany, each given to one of his sons.

Leir reigns for sixty years; upon reaching old age, he decides to divide his kingdom among his three daughters, Goneril, Regan and Cordelia. However, because of a slight, he gives Cordelia no land, whereupon she departs for Gaul. After Leir has had all his attendants taken from him, he begins to regret his actions towards Cordelia and travels to Gaul. Cordelia helps raise a Gaulish army for Leir, who returns to Britain, defeats his sons-in-law and regains the kingdom. Cordelia inherits the throne and rules for five years her sisters' sons, Cunedagius and Marganus, rebel against her. They imprison Cordelia; grief-stricken, she kills herself. Ending a civil war, Cunedagius eventually kills Marganus in Wales.

In a later era, Dunvallo Molmutius, the son of Cloten, the King of Cornwall, establishes his rule over the whole island. He is said to have established the Molmutine Laws. Dunvallo's sons, Belinus and Brennius, fight a civil war before being reconciled by their mother, and proceed to sack Rome. Victorious, Brennius remains in Italy, while Belinus returns to rule Britain. Numerous brief accounts of successive kings follow.

After his conquest of Gaul, Julius Caesar looks over the sea and resolves to order Britain to swear obedience and pay tribute to Rome. Cassivellaunus pays tribute and makes peace with Caesar, who then returns to Gaul. He dies and is succeeded by his nephew Tenvantius. Tenvantius' grandson Guiderius refuses to pay tribute to emperor Claudius, who then invades Britain. After Guiderius is killed in battle with the Romans, his brother Arvirargus eventually agrees to submit to Rome, and is given the hand of Claudius's daughter Genvissa in marriage.

The line of British kings continues under Roman rule. When Octavius passes the crown to his son-in-law Maximianus, his nephew Conan Meriadoc is given rule of Brittany to compensate him for not succeeding. After a long period of Roman rule, the Romans decide they no longer wish to defend the island and depart. The Britons are immediately besieged by attacks from Picts, Scots and Danes. In desperation the Britons send letters to the general of the Roman forces, asking for help, but receive no reply.