Obama "wished all Greeks well who participated in the democratic process. The president reiterated the importance of the US-Greece relationship and said he looks forward to working with Mr Papandreou", according to a statement released by the White House.
Papandreou's Socialist party won 44 percent of the votes, or 160 of 300 parliamentary seats, according to the interior ministry, which had counted 87 percent of the votes.
Papandreou's victory was seen as an expression of voters' discontent over the conservative government's handling of the economic crisis and corruption.
His father and grandfather also served as prime ministers in Greece over past decades.
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