According to the Finnish Justice Ministry, Niinisto ensured his re-election in the first round of presidential election, gaining 62.7 percent of votes while his rivals garnered less than 15 percent of ballots.
Niinisto, a former finance minister and parliament speaker, has been popular since taking office in 2012.
Pekka Haavisto, who also finished second in 2012, conceded the race long before the vote-count was completed, telling the Finnish national broadcaster that Niinisto "is the republic's new president with this result".
"We will follow very closely what is happening outside of Finland, globally, and if needed, then we will react".
Other candidates, including that of the Social Democrats, the party of Mr. Niinisto's predecessor, got less than 5 percent. He ran as an independent, with no association with the conservative National Coalition Party he once chaired.
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"I am surprised and touched by this big support", Niinisto told reporters in Helsinki, who secured over 62 per cent of the vote with almost all the ballots counted.
Jockeying for third place are the populist-nationalist Finns Party candidate Laura Huhtasaari - one of three women in the field - and veteran Centre Party politician Paavo Vayrynen who is running as an independent. He faced flak for inviting Putin to visit shortly after the annexation.
It has developed closer ties with North Atlantic Treaty Organisation but has not joined the alliance as it attempts to avoid antagonizing Moscow.
Nearly 4.5 million voters were eligible to vote for the president, a largely ceremonial post following changes to the constitution adopted in 1999. Yet while the president remains active in foreign policy, including holding the chairmanship of the Arctic Council since May 2017, presidential power over domestic policy has ebbed steadily since the final throes of the Cold War.