William Parr, Marquess of Northampton, Earl of Essex, 1st Baron Parr, 1st Baron Hart, was the only brother of Queen Catherine Parr, the sixth and final wife of King Henry VIII. He was a "sincere, plain, direct man, not crafty nor involved", whose "delight was music and poetry and his exercise war" who co-authored a treatise on hare coursing. He was in favour with Henry VIII and his son Edward VI, under whom he was the leader of the Protestant party, but having supported the desire of the latter to be succeeded by the Protestant Lady Jane Grey, was attainted by Edward's Catholic half-sister, Queen Mary I. He was restored by her Protestant half-sister, Queen Elizabeth I. He married thrice but died without issue.

Catherine Parr was Queen of England and Ireland as the last of the six wives of King Henry VIII from their marriage on 12 July 1543 until Henry's death on 28 January 1547. Catherine was the final queen consort of the House of Tudor, and outlived Henry by a year and eight months. With four husbands, she is the most-married English queen. She was the first woman to publish in print an original work under her own name, in English, in England.