Thomas, 2nd Earl of Lancaster was an English nobleman of the first House of Lancaster of the royal Plantagenet Dynasty. He was Earl of Lancaster, Leicester, and Derby from 1296 to 1322, and Earl of Lincoln and Salisbury jure uxoris from 1311 to 1322. As one of the most powerful barons of England, Thomas was one of the leaders of the baronial opposition to his first cousin, King Edward II.

The House of Lancaster was a cadet branch of the royal House of Plantagenet. The first house was created when King Henry III of England created the Earldom of Lancaster—from which the house was named—for his second son Edmund Crouchback in 1267. Edmund had already been created Earl of Leicester in 1265 and was granted the lands and privileges of Simon de Montfort, 6th Earl of Leicester, after de Montfort's death and attainder at the end of the Second Barons' War. When Edmund's son Thomas, 2nd Earl of Lancaster, inherited his father-in-law's estates and title of Earl of Lincoln he became at a stroke the most powerful nobleman in England, with lands throughout the kingdom and the ability to raise vast private armies to wield power at national and local levels. This brought him—and Henry, his younger brother—into conflict with their cousin King Edward II, leading to Thomas's execution. Henry inherited Thomas's titles and he and his son, who was also called Henry, gave loyal service to Edward's son King Edward III.