The SPO finished with 37.5 per cent of the votes, according to Liese Prokop, the interior minister, who read out the results on state TV on Sunday evening. The result does not yet include around 400,000 postal votes.
The People's Party (OVP), Austria's previous rulers, a centre-right Christian-oriented party, led by Wolfgang Schuessel, the previous chancellor, fell short with 34.2 per cent.
The results show a turn around in Austrian sentiment and have put the country on a centre-left course.
Gusenbauer became leader of the Social Democrats in 2000 when his predecessor Viktor Klima resigned after the party fell out of government for the first time since 1970.
Born on February 8, 1960, in the eastern Austrian town of St Poelten, Gusenbauer was the son of a construction worker and a cleaning woman.
He has been an active socialist since his school days and has long been involved in the disarmament movement.
Gusenbauer failed to oust Schuessel in the 2002 election. However, the SPO were celebrating this time around after running a successful campaign.