The Third Succession Act of Henry VIII's reign was passed by the Parliament of England in mid-1543, and returned both Mary and Elizabeth to the line of the succession behind Prince Edward. The Act was formally titled the Succession to the Crown Act 1543 (citation 35 Hen. VIII c.1), or the Act of Succession 1543; it is often incorrectly dated 1544. The Act superseded the First Succession Act and the Second Succession Act, which had left Prince Edward the only heir to the throne. It returned both of Henry's daughters Mary and Elizabeth to the line of succession, behind Edward, any potential children of his, and any potential children of Henry by his current wife Catherine Parr.

Edward VI attempted to ignore the Act in his will by naming Lady Jane Grey as his successor in place of Mary. However this failed and Mary took her rightful place on the throne as ordained by the Act.