Sir Thomas Mortimer (c. 1350–1399) was a medieval English soldier and statesman who served briefly in several important administrative and judicial state offices in Ireland and played a part in the opposition to the government of King Richard II. He was an illegitimate member of the Mortimer family, who were one of the leading noble houses of England and Ireland, and he helped to manage the Mortimer lands during the minority of the family heir, his nephew Roger, earl of March. Sir Thomas was also a close associate of the Lords Appellant, the powerful faction of nobles who opposed the administration of King Richard II.