The Southampton Plot of 1415 is a conspiracy to replace King Henry V with Edmund Mortimer, 5th Earl of March.

Mortimer is the great-grandson of Lionel of Antwerp, 1st Duke of Clarence, second surviving son of King Edward III, and his claim to the throne is thus superior to that of Henry V and his father, Henry IV, who derive their claim from Henry IV's father, John of Gaunt, 1st Duke of Lancaster, third surviving son of Edward III.

Moreover, Edmund Mortimer's father, Roger Mortimer, 4th Earl of March, had been widely considered heir presumptive to King Richard II, who had no issue, and Edmund Mortimer himself had been heir presumptive to Richard II while a young child.