Niinistö had been widely expected to run for president after losing to Tarja Halonen in the last presidential election, but had been coy about revealing his intentions after standing down from parliament at the last election. Earlier this month he told YLE that he had already decided whether or not he would run for president, but he refused to say what exactly he had decided.
He explained that the delay was intended to give time for the election and government formation process to run its course before he announced his intentions.
Niinistö has had two spells as Finland’s Finance Minister and one as Justice Minister, and worked for the European Investment Bank.
Pekka Haavisto will be the candidate put forward by the Green League, while the other parties will choose their candidates in the autumn.