The announcement sets the stage for a conclave next month to elect a successor to the 85-year-old.
The pope's brother, Georg Ratzinger, said the pontiff had been advised by his doctor not to take any more transatlantic trips and had been considering stepping down for months.
Georg Ratzinger said his brother was having increasing difficulty walking and that his resignation was part of a "natural process."
"His age is weighing on him," the 89-year-old said of his 85-year-old brother. "At this age my brother wants more rest."
The Pope announced his resignation in Latin during a meeting of Vatican cardinals, surprising even his closest collaborators, even though Benedict had made clear in the past he would step down if he became too old or infirm to do the job.
He called his choice "a decision of great importance for the life of the church."
The move allows the Vatican to hold a conclave before Easter to elect a new pope, since the traditional mourning time that would follow the death of a pope does not have to be observed.
It will also allow Benedict to hold great sway over the choice of his successor. He has already hand-picked the bulk of the College of Cardinals - the princes of the church who will elect the next pope - to guarantee his conservative legacy and ensure an orthodox future for the church.
There are several papal contenders in the wings, but no obvious front-runner - the same situation when Benedict was elected pontiff in 2005 after the death of Pope John Paul II.
The
Vatican stressed that no specific medical condition prompted Benedict's
decision, but in recent years, the pope has slowed down significantly,
cutting back his foreign travel and limiting his audiences. He now goes
to and from the altar in St Peter's Basilica on a moving platform, to
spare him the long walk down the aisle. Occasionally he uses a cane.
Benedict emphasised that carrying out the duties of being pope requires "both strength of mind and body."
"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 the
cardinals.
"In order to govern the barque of St Peter and
proclaim the Gospel, both strength of mind and body are necessary -
strengths which in the last few months, have deteriorated in me to the
extent that I have had to recognise my incapacity to adequately fulfil
the ministry entrusted to me," he said.
Popes are allowed
to resign; church law specifies only that the resignation be "freely
made and properly manifested." But only a handful have done it.
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 most famous resignation was Pope Celestine V in 1294; Dante placed
him in hell for it.
When Benedict was elected aged 78, he
was the oldest pope chosen in nearly 300 years. He had already been
planning to retire as the Vatican's chief orthodoxy watchdog to spend
his final years writing in the "peace and quiet" of his native Bavaria.
Benedict
said he would serve the church for the remainder of his days "through a
life dedicated to prayer."
The Vatican said immediately after his
resignation, Benedict would go to Castel Gandolfo, the papal summer
retreat south of Rome, and then would live in a cloistered monastery.
Contenders
to be his successor include Cardinal Angelo Scola, archbishop of Milan,
Cardinal Christoph Schoenborn, the archbishop of Vienna, and Cardinal Marc Ouellet, the Canadian head of the Vatican's office for bishops.
Longshots
include Cardinal Timothy Dolan of New York. Although he is popular and
backs the pope's conservative line, the general thinking is that the
Catholic Church does not need a pope from a "superpower".
Given
half of the world's Catholics live in the global south, there will once
again be arguments for a pope to come from the developing world.
Cardinal Antonio Tagle, the archbishop of Manila, has impressed many Vatican watchers, but at 56 and having only been named a cardinal last year, he is considered too young.
Cardinal
Peter Kodwo Appiah Turkson of Ghana is one of the highest-ranking
African cardinals at the Vatican, currently heading the Vatican's office
for justice and peace, but he is something of a wild card.
All cardinals under 80 are allowed to vote in the conclave, the secret meeting held in the Sistine Chapel
where cardinals cast ballots to elect a new pope.
As per tradition, the
ballots are burned after each voting round; black smoke that snakes out
of the chimney means no pope has been chosen, while white smoke means a
pope has been elected.
The pontiff had been due to attend World Youth Day in July in Rio de Janeiro; by then his successor will have been named and will presumably make the trip.
Benedict
himself raised the possibility of resigning if he were simply too old
or sick to continue on, when he was interviewed in 2010 for the book
"Light of the World."
"If a pope clearly realises that he
is no longer physically, psychologically and spiritually capable of
handling the duties of his office, then he has a right, and under some
circumstances, also an obligation to resign," Benedict said.
The former Cardinal Joseph Ratzinger
had an intimate view as Pope John Paul II, with whom he had worked
closely for nearly a quarter-century, suffered through the debilitating
end of his papacy.
The announcement took the Vatican - and the rest of the world - by surprise.
Several
cardinals did not even understand what Benedict had said during the
consistory, said the Rev. Federico Lombardi, the Vatican spokesman.
Others who did were stunned.
"All the cardinals remained
shocked and were looking at each other," said Monsignor Oscar Sanchez of
Mexico who was in the room when Benedict made his announcement.
Benedict
was born on April 16, 1927 in Marktl Am Inn, in Bavaria, but his
father, a policeman, moved frequently and the family left when he was 2.
In
his memoirs, Benedict dealt what could have been a source of
controversy had it been kept secret - that he was enlisted in the Nazi
youth movement against his will when he was 14 in 1941, when membership
was compulsory.
He said he was soon let out because of his studies for
the priesthood. Two years later he was drafted into a Nazi anti-aircraft
unit as a helper. He deserted the German army in April 1945, the waning days of the war.
He called it prophetic that a German followed a Polish pope - with both men coming from such different sides of the Second World War.
Benedict
was ordained, along with his brother, in 1951.
After spending several
years teaching theology in Germany, he was appointed bishop of Munich in
1977 and elevated to cardinal three months later by Pope Paul VI.
John Paul named him leader of the Congregation for the Doctrine of the Faith in 1981 and he took up his post a year later.