Katherine Brandon, Duchess of Suffolk, suo jure 12th Baroness Willoughby de Eresby (née Willoughby; 22 March 1519 – 19 September 1580), was an English noblewoman living at the courts of King Henry VIII, King Edward VI, Queen Mary I and Queen Elizabeth I. She was the fourth wife of Charles Brandon, 1st Duke of Suffolk, who acted as her legal guardian during his third marriage to Henry VIII's sister Mary. Her second husband was Richard Bertie, a member of her household. Following Charles Brandon's death in 1545, it was rumoured that King Henry had considered marrying Katherine as his seventh wife, while he was still married to his sixth wife, Catherine Parr, who was Katherine's close friend.
An outspoken supporter of the English Reformation, she fled abroad to Wesel and later the Grand Duchy of Lithuania during the reign of the Catholic Queen Mary I, to avoid persecution.
