The pope is to resign his office after declaring he was too old to carry on as head of the Catholic Church.
Pope Benedict XVI will quit on February 28, the Vatican said, adding the announcement was a surprise.
A Vatican statement said the pope was unable to continue in office due to his age and diminishing strength, he is 85, and the papacy will remain vacant until a successor is elected.
He is the first pontiff to resign in nearly 600 years. The decision sets the stage for a conclave to elect a new pope before the end of March.
The pope announced his decision in Latin during a meeting of Vatican cardinals.
"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," he told them. "I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only by words and deeds but no less with prayer and suffering.
"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 - strengths which in the last few months, has deteriorated in me to the extent that I have had to recognise my incapacity to adequately the ministry entrusted to me."
The last pope to resign was Pope Gregory XII, who stepped down in 1415 in a deal to end the Great Western Schism among competing papal claimants.
The pope called his choice "a decision of great importance for the life of the church."
There are several papal contenders in the wings, but no obvious front-runner as was the case when Pope Benedict was elected pontiff in 2005 after the death of Pope John Paul II.
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