Barbara Palmer, 1st Duchess of Cleveland (27 November [O.S. 17 November] 1640 – 9 October 1709), more often known by her maiden name Barbara Villiers or her title of Countess of Castlemaine, was an English royal mistress of the Villiers family and perhaps the most notorious of the many mistresses of King Charles II of England, by whom she had five children, all of them acknowledged and subsequently ennobled. Barbara was the subject of many portraits, in particular by court painter Sir Peter Lely. In the Gilded Age, it was stylish to adorn an estate with her likeness.
Barbara's first cousin Elizabeth Villiers (later 1st Countess of Orkney 1657–1733) was the presumed mistress of King William III.
Born into the Villiers family as Barbara Villiers, in the parish of St. Margaret's, Westminster, Middlesex, she was the only child of William Villiers, 2nd Viscount Grandison, a half-nephew of the 1st Duke of Buckingham, and of his wife Mary Bayning, co-heiress of Paul Bayning, 1st Viscount Bayning. On 29 September 1643 her father died in the First English Civil War from a wound sustained on 26 July at the storming of Bristol, while leading a brigade of Cavaliers. He had spent his considerable fortune on horses and ammunition for a regiment he raised himself; his widow and daughter were left in straitened circumstances. Shortly after Grandison's death, Barbara's mother married secondly Charles Villiers, 2nd Earl of Anglesey, a cousin of her late husband.