Katherine Brandon, Duchess of Suffolk, suo jure 12th Baroness Willoughby de Eresby, 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 Katherine as his seventh wife, while he was still married to his sixth wife, Catherine Parr, who was Katherine's close friend.

Suo jure is a Latin phrase, used in English to mean 'in his own right' or 'in her own right'. In most nobility-related contexts, it means 'in her own right', since in those situations the phrase is normally used of women; in practice, especially in England, a man rarely derives any style or title from his wife although this is seen in other countries when a woman is the last heir of her line. It can be used for a male when such male was initially a 'co-lord' with his father or other family member and upon the death of such family member became the sole ruler or holder of the title "in his own right" (alone).