Roger de Mortimer, 3rd Baron Mortimer, 1st Earl of March (April 25 , 1287 – November 29, 1330), is an English nobleman and powerful Marcher lord who has gained many estates in the Welsh Marches and Ireland following his advantageous marriage to the wealthy heiress Joan de Geneville, 2nd Baroness Geneville.

In November 1316, he is appointed Lord Lieutenant of Ireland.

He is imprisoned in the Tower of London in 1322 for having led the Marcher lords in a revolt against King Edward II in what becomes known as the Despenser War.

He later escapes to France, where he is joined by Edward's queen consort Isabella, whom he takes as his mistress.

After he and Isabella lead a successful invasion and rebellion, Edward is subsequently deposed; Mortimer allegedly arranges his murder at Berkeley Castle.

For three years, Mortimer is de facto ruler of England before being himself overthrown by Edward's eldest son, Edward III.

Accused of assuming royal power and other crimes, Mortimer is executed by hanging at Tyburn.