Efforts were underway for a year to draft Haavisto as a candidate, but it was not until Wednesday that he confirmed that he would seek the presidency.
"A year ago I expressed the hope that a campaign would take off among a wider group than the Greens. That hope has now been fulfilled," Haavisto said in thanking supporters.
Haavisto's backers include a number of Finland's leading writers, academics and musicians.
"Miracles happen when you made them happen. The biggest surprise in the presidential election would be that there were no surprises. Today, we can start a surprise together," Haavisto added.
According to Haavisto, Finland should no longer dream of a constitutionally strong presidency. He stressed that one of the main jobs of the president is to raise awareness and understanding of Finland in the world.
The Green League is expected to nominate Haavisto, a former minister, Greens chair and UN official, as its candidate at a party congress this weekend.
