Aso, 68, won 351 of 525 votes cast by LDP members of parliament and representatives of local party branches.
The former foreign minister was among five candidates bidding for the LDP leadership and is all but guaranteed to replace Prime Minister Yasuo Fukuda, who unexpectedly stepped down three weeks ago amid dwindling approval ratings over the poor performance of the world's second-largest economy.
Japan's lower house, where the LDP-led coalition enjoys an overwhelming majority, will vote in a new prime minister on Wednesday.
After assuming the premiership, Aso will face the difficult task of reviving the economy, which is burdened by enormous public debt. As part of his election campaign, he proposed tax cuts for businesses and investors.
Experts believe, though, that he will most likely stick to the foreign policy conducted by his predecessor, strengthening Japan's security alliance with the United States and improving relations with China.
