Mr Aso is set become Japan's first Catholic prime minister
Members of Japan's ruling party have selected a bluff conservative, Taro Aso, as their new leader, meaning he is almost certain to become the next PM.
Mr Aso advocates greater public spending to promote the economy, and an assertive foreign policy.
He overwhelmed his four Liberal Democratic Party rivals for the party leadership in a crowded race.
The LDP is struggling to combat a long-term slump in public popularity, and early general elections are now likely.
Its last two prime ministers have quit after serving just a year each.
The LDP's recent slump in popularity has led some to suggest that the party could be on the verge of losing power - an almost unthinkable prospect for most of the past 50 years.
The party now hopes Mr Aso's brash straight-talking style and charisma - he is known for his love of adult manga cartoons - will prove an antidote to the opposition's rising popularity.
Mr Aso received 351 out of the 527 votes cast by MPs and members of regional chapters, party election chief Hideo Usui announced after voting.
Kaoru Yosano, the minister for economic and fiscal policy, trailed in second place at 66 votes. Former defence minister Yuriko Koike, who hoped to become Japan's first female prime minister, placed third with 46 votes.
The LDP leader looks certain to be approved as the country's prime minister on Wednesday because of the party's majority in the lower house of parliament. But Mr Aso is widely predicted to put his leadership quickly to the test with a general election.
"Standing here, I feel that this is Taro Aso's destiny," Mr Aso - the grandson of a prime minister - told LDP members after the vote, according to Reuters news agency.
"But the LDP, as the government party, must resolutely fight the [opposition] Democratic Party in the next election, and only when we have won that election will I have fulfilled my destiny."
Mr Aso called for unity among the five contenders for the LDP leadership, and reports suggest some of them may feature in his cabinet once he is confirmed as prime minister.
He will face the uphill challenge of steering the Japanese economy away from the brink of recession.
The 68-year-old veteran is promising greater public spending to try to stimulate the economy - particularly in rural areas, where the party is traditionally strong.
Before the vote Mr Aso, a former foreign minister, pledged to a crowd of supporters in Tokyo that he would sort out Japan's economy.
"America is facing a financial crisis... we must not allow that to bring us down as well," he said. The US is a leading market for Japan-made cars, electronics and other exports.
But his leadership rival Mr Yosano accused him of risking Japan's long-term interests through wasteful spending.
Other colleagues fear higher spending could mark a return to the old profligate ways of the LDP, where expensive public works projects were used to try to create jobs, hollowing out the public finances.
Japan's last prime minister, Yasuo Fukuda, abruptly resigned last September after less than a year in the job, having fought chronic low approval ratings and political deadlock caused by the opposition's popularity.
The main opposition Democratic Party (DPJ) made big gains in recent elections and controls the upper house of parliament.
At its own convention on Sunday, the DJP gave another two-year term to leader Ichiro Ozawa.