Katherine Seymour, Countess of Hertford (formerly Katherine Herbert, Lady Herbert, born Lady Katherine Grey; 25 August 1540 – 27 January 1568), was a younger sister of Lady Jane Grey. A granddaughter of Henry VIII's sister Mary, she emerged as a prospective successor to her cousin, Elizabeth I of England, before incurring Queen Elizabeth's wrath by secretly marrying Edward Seymour, 1st Earl of Hertford. Arrested after the Queen was informed of their clandestine marriage, Katherine (as Lady Hertford) lived in captivity until her death, having borne two sons in the Tower of London.