Sir Edmund Mortimer (10 December 1376 – 1409), who has a claim to the crown through his mother, Philippa Plantagenet, plays a part in the rebellions of the Welsh leader, Owain Glyndŵr, and the Percys, and perishes at the siege of Harlech.

Edmund Mortimer, born 10 December 1376 at Ludlow Castle in Shropshire, is the second son of Edmund Mortimer, 3rd Earl of March, by his wife Philippa Plantagenet.

He is a grandson of Lionel of Antwerp, 1st Duke of Clarence, and thus a great-grandson of King Edward III.

He has an elder brother, Roger Mortimer, 4th Earl of March, and two sisters, Elizabeth, who marry Henry 'Hotspur' Percy, and Philippa (1375-1401), who marries firstly John Hastings, 3rd Earl of Pembroke (d.1389), secondly Richard Fitzalan, 11th Earl of Arundel (1346-1397), and thirdly, Sir Thomas Poynings.

Edmund is financially well provided for, both by his father, who dies when Edmund is 5 years old, and by his elder brother, Roger.