By Roland Buerk
BBC News, Tokyo
Mr Hatoyama has promised deep changes in the way Japan is run
Japan is now beginning a process it has only been through once before since 1955 - the transition of power from the Liberal Democratic Party to a new government.
By morning the scale of the win was clear. Yukio Hatoyama's Democratic Party of Japan had 308 seats to the LDP's 119, an almost exact reversal of their standings before Sunday's general election.
Mr Hatoyama, the heir to an industrial and political dynasty often compared to the Kennedys, must now tackle Japan's deep-seated problems.
The economy has returned to growth after its worst post-war recession but is struggling to find a sustainable recovery.
Figures out on Monday were mixed, showing retail sales down, but industrial production up.
Unemployment is at record highs. Japan's national debt is pushing towards double its gross domestic product.
Despite that the next prime minister must try to fulfil his pledge to expand welfare payments, including an allowance for families of around $275 (£170) per month per child.
It is meant to encourage women to have more babies.
Japan's society is rapidly ageing and the population is in decline, a grave threat to the country's long term future.
Mr Hatoyama has also raised high expectations that he will change the way Japan is run.
For decades a so-called "iron triangle" of the Liberal Democratic Party, big business and the bureaucracy steered Japan from defeat in World War II to economic might.
People have handed Mr Hatoyama a thumping majority and the legislative clout to push change through parliament
The next prime minister has vowed to curb the power of the bureaucracy, perhaps an ambitious aim for a novice and untested government.
In foreign relations the Democratic Party of Japan have said they want to be more independent of the United States on diplomatic issues.
They have stressed though that the alliance remains critical and they want to keep relations good, while strengthening ties with their Asian neighbours.
After three Liberal Democratic Party prime ministers in as many years, all perceived as inept, the people of Japan were desperate for a new beginning.
The vote was as much an expression of disgust with the Liberal Democratic Party as an endorsement of the Democratic Party of Japan.
But the people have handed Mr Hatoyama a thumping majority and the legislative clout to push change through parliament.
His challenge now is not to disappoint.