As a man's religious convictions can be considered fundamental to his character and behaviour, the nature of King Richard III's piety has been the subject of considerable debate. Much of this controversy has focused on the book of hours adopted by the king for his own private use following his coronation, and to which certain prayers, including that known as the "Prayer of Richard III", were added. This study explores the manuscript and the Prayer's text. The manuscript (now preserved in Lambeth Palace Library) was originally produced in London around 1420. The text shows the preoccupations of a devout man of the 15th century and its decoration puts it in the context of the development of London manuscript illumination of the period. Moreover, in this analysis of the manuscript, the authors offer an insight into the personality of Richard III, one of the most controversial figures in the history of medieval Britain.