It is the first time in memory, and possibly centuries, that a pontiff has voluntarily stepped down.
Benedict, 85, who had appeared increasingly frail in recent months, said that his infirmity led him to the decision.
"After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry," the pope said in a surprise statement while at a ceremony to canonize three saints, according to a text released by the Vatican.
"I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering," Benedict said. "However, in today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of St. Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me.
"For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of St. Peter, entrusted to me by the Cardinals on 19 April 2005.
Benedict, who was Cardinal Joseph Ratzinger from Germany before his elevation to head the worldwide Roman Catholic Church, is the 265th pope in the church's history.
His resignation will set off weeks of speculation about a successor as the Vatican sets up a conclave of cardinals to elect the new pope.