Katherine Brandon, Duchess of Suffolk, suo jure 12th Baroness Willoughby de Eresby (22 March 1519 – 19 September 1580), was an English noblewoman living at the courts of King Henry VIII, King Edward VI 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 Catherine 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 Queen Mary I, to avoid persecution.
Katherine Willoughby, born at Parham Old Hall, Suffolk, on 22 March 1519 and christened in the church there four days later, was the daughter of William Willoughby, 11th Baron Willoughby de Eresby, and his second wife, María de Salinas. Lord Willoughby's first wife, Mary Hussey, the daughter of William Hussey, Chief Justice of the King's Bench, had died childless before 1512, and in June 1516 he married Maria de Salinas. Dona Maria de Salinas had come to the English court with Henry VIII's Queen consort, Catherine of Aragon, and was one of the Queen's ladies-in-waiting and closest friends. The King favoured another match bolstering his own marital alliance with Spain, and even named one of his warships the Mary Willoughby. It seems Katherine was named for the Queen, but her mother's lifelong friendship with Catherine of Aragon did not prevent her daughter from becoming one of England's Marian exiles later in life.