His grandfather even helped to create the once-unstoppable Liberal Democratic Party (LDP), which has ruled for most of the past half century, and Hatoyama himself belonged to the LDP until defecting in the 1990s.
The soft-spoken, Stanford-trained engineer, whose Democratic Party of Japan (DPJ) won a landslide victory in Sunday's general election, has promised "revolutionary change" in this traditionally risk-averse country.
The political blue-blood has vowed to chart a new course for the world's number two economy away from unrestrained US-style capitalism towards a kinder, more fraternal society.
Hatoyama, 62, has pledged to put the interests of ordinary people before those of big business and the powerful bureaucracy, and to pursue a less subservient relationship with the United States.
He has promised cash allowances for child-raising, free high-school education, an higher minimum wage, petrol tax cuts and an end to highway tolls.
While the DPJ has never ruled, Hatoyama does have some brief experience in government, having served as a deputy chief cabinet secretary in the early 1990s after the LDP was ousted from power for less than a year.
Politics runs deep in Hatoyama's family, which is sometimes likened to the Kennedys of the United States.
His father was a foreign minister in the 1970s and his brother was a cabinet minister under outgoing premier Taro Aso until earlier this year. His other grandfather founded Bridgestone, the world's largest tyremaker.
With his swept-back wavy hair, Hatoyama -- a former assistant professor -- has the air of an eccentric academic about him and was nicknamed the "alien" by his fellow party members for his quirky appearance.
He was first elected to the lower house of parliament in 1986 with a seat in Hokkaido, the northernmost of Japan's four main islands.
Hatoyama was elected in May as president of the DPJ -- which he co-founded over a decade ago -- for a second time, after his predecessor Ichiro Ozawa resigned over a political-donations scandal.
Hatoyama himself came under fire in late June for the accounting irregularities of his fund-raising body.
He said 21 million yen (224,000 dollars) had been wrongly recorded since 2005, an admission that followed media reports that the donors' list included names of dead people, as well as citizens who had denied giving him money.
Hatoyama is married to a former actress, Miyuki, who is known for her extrovert and quirky personality and has appeared on TV variety shows to discuss topics ranging from spirituality to politics.
They have one son, an engineering scholar now living in Russia.
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