Philippa Plantagenet, (16 August 1355 – 5 January 1380/1381), Countess of Ulster suo juris, was the daughter and only child of Lionel of Antwerp, 1st Duke of Clarence and Elizabeth de Burgh, 4th Countess of Ulster. Her father was the second son of King Edward III of England by his Queen consort Philippa of Hainault.

Philippa married Edmund Mortimer, 3rd Earl of March in 1368, an alliance that would have far-reaching consequences in English history. As a result of her seniority in the line of succession to the throne of England and her marriage into the powerful Mortimer family, her descendants eventually succeeded to the throne as the House of York under Edward IV.

She had five children by her marriage to Edmund Mortimer:
• Elizabeth de Mortimer (12 February 1370/1371 - 20 April 1417). She married firstly Henry Percy and secondly Thomas Camoys and had a son called Lord Roger de Camoys.
• Roger Mortimer, 4th Earl of March (11 April 1374- 20 July 1398).
• Philippa de Mortimer (21 November 1375 - 24 September 1401). She married first Sir John Hastings c. 1385, secondly Sir Thomas Poynings of Basing and thirdly Richard Fitzalan, 11th Earl of Arundel.
• Edmund Mortimer (9 November 1376 - 1409?). He married Catrin (Catherine) Glyndwr the daughter of Owain Glyndwr.
• Sir John de Mortimer (c. 1378 - 1424).