Phillipa Plantagenet, (16 August 1355 - 5 January 1380/1381), Countess of Ulster suo juris, was the daughter and only child of Lionel of Antwerp, Duke of Clarence, the third son of King Edward III of England by his wife Elizabeth de Burgh (d. December 10, 1363).

She 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.

Children

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 .
• 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?).
• Sir John de Mortimer (c. 1378 - 1424).