Historia regum Britanniae, originally called De gestis Britonum, is a pseudohistorical 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.

Pseudohistory is a form of pseudoscholarship that attempts to distort or misrepresent the historical record, often by employing methods resembling those used in scholarly historical research. The related term cryptohistory is applied to pseudohistory derived from the superstitions intrinsic to occultism. Pseudohistory is related to pseudoscience and pseudoarchaeology, and usage of the terms may occasionally overlap. Although pseudohistory comes in many forms, scholars have identified many features that tend to be common in pseudohistorical works; one example is that the use of pseudohistory is almost always motivated by a contemporary political, religious, or personal agenda. Pseudohistory also frequently presents sensational claims or a big lie about historical facts which would require unwarranted revision of the historical record.