Born into a noble family (his father was the governor of Alcantara in Spain), Peter studied law at Salamanca University and, at 16, joined the so-called Observant Franciscans (also known as the discalced, or barefoot, friars). While he practiced many penances, he also demonstrated abilities which were soon recognized. He was named the superior of a new house even before his ordination as a priest; at the age of 39, he was elected provincial; he was a very successful preacher. Still, he was not above washing dishes and cutting wood for the friars. He did not seek attention; indeed, he preferred solitude.
Peters penitential side was evident when it came to food and clothing. It is said that he slept only 90 minutes each night. While others talked about Church reform, Peters reform began with himself. His patience was so great that a proverb arose: "To bear such an insult one must have the patience of Peter of Alcantara."
In 1554, Peter, having received permission, formed a group of Franciscans who followed the Rule of St. Francis with even greater rigor. These friars were known as Alcantarines. Some of the Spanish friars who came to North and South America in the 16th, 17th and 18th centuries were members of this group. At the end of the 19th century, the Alcantarines were joined with other Observant friars to form the Order of Friars Minor.
As spiritual director to St. Teresa, Peter encouraged her in promoting the Carmelite reform. His preaching brought many people to religious life, especially to the Secular Franciscan Order, the friars and the Poor Clares.
He was canonized in 1669.