"Power plants that went into operation before the end of 1980 will... be shut down for the period of the moratorium," Merkel told a news conference. The nuclear issue should be addressed at an EU summit on March 24-25, she added.
Environment Minister Norbert Roettgen said it was not clear if all nuclear power plants shut down during the three-month moratorium would remain closed or be reconnected to the grid afterwards.
Merkel astonished German politicians by suspending an unpopular coalition decision taken only last autumn, under which the life of Germany's 17 nuclear power plants would be extended by years.
She drew accusations of transparent trickery for the move, with the opposition and media saying she was trying to avoid a regional election disaster later this month.
Merkel announced the closures after a meeting with the premiers of every German state with a nuclear plant.
Last year the government had decided to keep the nuclear plants — operated by E.ON, RWE, EnBW and Vattenfall — running for about 12 years beyond their original shutdown date, despite protests even before the earthquake and tsunami hit Japan.
Her policy change drew cynicism from the opposition. "She just wants to get through the provincial assembly elections," said Social Democrat leader Sigmar Gabriel, accusing her of playing political tactics with people's fears.
"The whole thing doesn't make sense and is really just a transparent trick," he told ARD television.
Merkel faces three regional elections in the next fortnight, including in the wealthy southwestern state of Baden-Wuerttemberg, which has long been a stronghold of Merkel's Christian Democrats CDU.
Even before the Japanese crisis the CDU faced losing control in Baden-Wuerttemberg, which would be a political disaster for Merkel. Last month her party suffered a thrashing in elections in Hamburg, Germany's richest city.