Roger de Mortimer, 4th Earl of March and 6th Earl of Ulster (11 April 1374 – 20 July 1398) is a fourteenth-century English nobleman.

He is considered the heir presumptive to King Richard II.

Roger Mortimer's father, the 3rd Earl of March, dies in 1381, leaving a young heir.

His mother, Philippa, a granddaughter of Edward III, had already died several years earlier.

The six-year-old Roger succeeds to his father's title and is placed under the wardship of Thomas Holland, 2nd Earl of Kent, eventually marrying Holland's daughter Alianore.

During his life Mortimer spends much time in Ireland; he serves several tenures as Lord Lieutenant of Ireland and dies during a skirmish in Kells.

He is succeeded by his young son, Edmund Mortimer, 5th Earl of March.