Pope Benedict XVI announced during a mass Monday plans to step down on February 28, citing the job's pressures and advanced age, dpa reported.
"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 said.
The announcement came during a Latin-language ceremony. He said he had long thought about his decision, which he said was for the good of the church.
The pope's brother, Georg Ratzinger, told dpa that Benedict - born Joseph Ratzinger - was stepping down for health reasons.
"His age is weighing him down," said Georg Ratzinger.
The pope, 85, has held the highest office in the Catholic church since 2005. He has long been hounded by rumours that he might step down for health reasons.
It is unusual for a pope to resign, as the job is considered a lifetime duty. However, it is not without precedent.
The comments came during a consistory meeting, in which several prominent deceased Catholics were expected to be canonized for sainthood.