Japan's parliament has backed Yoshihiko Noda as the country's sixth prime minister in five years.
The vote came after the 54-year-old former finance minister secured the leadership of the ruling Democratic Party in an election on Monday.
Ex-PM Naoto Kan, criticised for his handling of the March quake, formally resigned with his cabinet beforehand.
Correspondents say the new PM faces a daunting agenda, including trying to unify a divided party.
Large parts of Japan need to be rebuilt after March's earthquake and tsunami, and the crisis at the crippled Fukushima nuclear power plant still needs to be resolved.
Added to that, Mr Noda, a fiscal conservative, will need to address Japan's stagnant economy.
He has said in the past that he favours raising funds through increased taxation - including a doubling of Japan's sales tax, which currently stands at 5% - to cut debt and meet social security commitments.
On Tuesday he said Japan faced problems with the high yen and with deflation.
"On the topic of fiscal discipline, we need to carry out careful management of the economy and public finances," he said.
Unlike Mr Kan, he wants Japan's halted nuclear reactors to be restarted and has not backed his call for a nuclear-free Japan.
Monday's vote went to a run-off between Mr Noda and Banri Kaieda, the former trade minister backed by party heavyweight Ichiro Ozawa, after no candidate secured a majority in the first round.
Mr Noda won the run-off after lawmakers backing the public's choice, former foreign minister Seiji Maehara, swung behind him.
The DPJ won power in a general election in 2009, ending half a century of almost unbroken rule by the Liberal Democratic Party.
But it lost control of the upper house in polls in 2010 and has struggled to pass key legislation through parliament.
It has also been hit by in-fighting, with the leadership race turning into a bitter factional battle between supporters and enemies of Mr Ozawa.
Known as the Shadow Shogun, Mr Ozawa commands the loyalty of around 130 lawmakers, despite awaiting trial on charges of misreporting political donations. He is currently suspended from the party.