Amadeus VIII (4 September 1383 – 7 January 1451), nicknamed the Peaceful, was Count of Savoy from 1391 to 1416 and Duke of Savoy from 1416 to 1440. He was the first to hold the ducal title, granted by Emperor Sigismund. Known for his diplomatic temperament and administrative reforms, he strengthened the state's institutions and fostered internal peace.

He also claimed the papacy from 1439 to 1449 as Antipope Felix V, in opposition to Popes Eugene IV and Nicholas V, becoming the last historical antipope recognised by a significant portion of the Catholic clergy.

Amadeus was born in Chambéry on 4 September 1383, the son of Amadeus VII, Count of Savoy and Bonne of Berry. Even as a boy, he suffered from strabismus, which one of his father's physicians, Jean de Grandville, claimed he could cure. He became count of Savoy in 1391 after his father's death on 2 November 1391, caused (it was said) by poisoning, or at least bad treatment at the hands of his physicians. In his testament, the Red Count, Amadeus VII, had appointed his mother Bonne de Bourbon rather than his wife Bonne de Berry, niece of King Charles VI of France, as regent for Amadeus VIII. This set off a struggle at the court between the grandmother and the mother, eventually involving the dukes of Berry, Bourbon, Orléans, and King Charles VI of France. Amadeus VIII's grandmother acted as regent until 1397, during his minority.

The Count's early rule saw the centralisation of power and the territorial expansion of the Savoyard state, and in 1416 Amadeus was elevated by Emperor Sigismund to duke of Savoy. In 1418, his distant cousin Louis of Piedmont, his brother-in-law, the last male of the elder branch of House of Savoy, died, leaving Amadeus as his heir-general, thus finally uniting the male lines of the House of Savoy.

Amadeus increased his dominions and encouraged several attempts to negotiate an end to the Hundred Years' War. From 1401 to 1422, he campaigned to recover the area around Geneva and Annecy. After the death of his wife in 1428, he founded the Order of Saint Maurice with six other knights in 1434. They lived alone in the castle of Ripaille, near Geneva, in a quasi-monastic state according to a rule drawn up by himself. He styled himself Decanus Militum solitudinis Ripalliae. He appointed his son Louis regent of the duchy.

Amadeus was sympathetic to conciliarism, the movement to have the Church managed by Ecumenical councils, and to prelates like Cardinal Aleman of Arles, who wanted to set limits upon the doctrine of papal supremacy. He had close relations with the Council of Basel (1431–1449), even after most of its members joined the Council of Florence, convened by Pope Eugene IV in 1438.

There is no evidence that he was intrigued to obtain the papal office by sending the bishops of Savoy to Basel, though he did suggest that the bishops of Savoy attend the Council. Of the twelve bishops present, seven were Savoyards.

In its Session XXXI, on 24 January 1438, the Council of Basel suspended Pope Eugene. Then, on 25 June 1439, it formally deposed Eugene as a heretic. The president of the Council, Cardinal Louis Aleman, the archbishop of Arles, reminded the members that they needed to elect a rich and powerful pope to defend it from its adversaries.