According to the preliminary results, Van der Bellen has won 53,6 percent of votes, while his rival Norbert Hofer received 46,4 percent with 56 percent of the ballots counted by 17:05 local time (16:05 GMT).
The official results of the of the Austrian presidential election rerun will be announced by Monday evening after the counting of votes is completed, Austrian Interior Minister Wolfgang Sobotka said earlier in the day.
In the early part of his political career Van der Bellen, now 72 years old, belonged to the Social Democrats, but later became a member of the Green Party, being its lawmaker for 18 years and heading the party in 1997-2008, when it was fourth-largest force in the Parliament.
Alexander Van der Bellen, whose father was born in Russia and mother in Estonia and who has often called himself "a child of refugees," has been critical of Austria’s current asylum and migrant policy that was toughened by the ruling coalition in response to the migrants’ influx.
He also said that he was against the free trade deals such as TTIP and CETA.
