"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," Benedict, 85, who was elected pope April 19, 2005, said in a statement.
"[In] today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith ... both strength of mind and body are necessary ... 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," the pope said.
The pope's announcement caught church officials by surprise, ANSA reported.
Italian Cardinal Angelo Sodano, dean of the College of Cardinals, said Benedict's decision was a "bolt out of the blue."
Acknowledging the gravity of his decision, the pontiff said, "I renounce the ministry of bishop of Rome, successor of Saint Peter, entrusted to me by the cardinals on [April 19, 2005], in such a way, that as from [Feb. 28, 2013,] at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a conclave to elect the new supreme pontiff will have to be convoked ... ."
He thanked the papal community for its support and asked pardon "for all my defects."
Benedict said he wanted to "devotedly serve the holy church of God in the future through a life dedicated to prayer."
"The Holy Father brought the tender heart of a pastor, the incisive mind of a scholar and the confidence of a soul united with his God in all he did," Cardinal Timothy Dolan of New York, president of the U.S. Conference of Catholic Bishops, said in a statement. "We are sad that he will be resigning but grateful for his eight years of selfless leadership ... ."
U.S. President Obama said he and his wife, Michelle, "wish to extend our appreciation and prayers to His Holiness Pope Benedict XVI" on behalf of all Americans.
"The Church plays a critical role in the United States and the world, and I wish the best to those who will soon gather to choose [the pope's] successor," Obama said.
Mohammed Shafiq, chief executive officer the Ramadhan Foundation in Manchester, England, said Benedict's papacy would be remembered by the Muslim world for his "his distortion and attack on Islam" that "tarnished" the work of his predecessor, Pope John Paul II.
"The Catholic Church now has a chance to return back to the teachings and practices of Pope John Paul II, which were of interfaith work and respect for our respective positions," Shafiq said, "and I hope that once a new pope is elected we actually see our faiths come together.
Benedict, the former Cardinal Joseph Ratzinger, is considered a doctrinal conservative. Among his stands included the assertion that Catholicism is "true" and that other religions are "deficient," The New York Times reported. His other well-known stands include a view that the modern, secular world -- particularly in Europe -- is spiritually weak; and that Catholicism is in competition with Islam.
The pope opposed homosexuality, the ordination of women priests and stem cell research.
Benedict is the 265th pope.
Pope Celestine V in 1294 resigned in 1294. Pope Gregory XII abdicated in 1415 to end what was known as the Western Schism among competitors for the papacy.