“We got the biggest mandate,” said the leader of the National Coalition Party, after a dramatic night in which the lead gradually swung away from Ms Marin’s Social Democrats.
Mr Orpo was heading for 20.8% of the vote, ahead of the right-wing populist Finns Party and the centre left.
It is a bitter defeat for Ms Marin.
Shortly after the conservative leader claimed victory, she conceded the election even though her party was set to increase its share of seats in parliament.
For weeks it had been seen as a three-way race, and as the results came in it became too close to call. Then a projection from public broadcaster YLE gave Petteri Orpo’s National Coalition victory the biggest number of seats in parliament.
Finns Party leader Riikka Purra congratulated her rival and was overjoyed to have secured the best result in her party’s history. She was heading for 20% of the vote with almost all the votes counted, while Ms Marin’s Social Democrats were on course for 19.9%.
Sanna Marin became the world’s youngest leader when she burst on to the political scene in 2019. She headed a coalition of five parties, all led by women.
Now 37, she has steered Finland to within days of Nato membership, having won plaudits for her country’s response to neighbouring Russia’s full-scale invasion.
Her poll ratings are still high, but the election was largely fought on Finland’s public debt as all the mainstream parties backed Nato membership.
Many Finns see her as a polarising figure. She came under heavy scrutiny last year when a video emerged of her singing, dancing and drinking at a party. Supporters said the controversy was steeped in sexism and women across Finland and the world shared videos of themselves dancing in solidarity.
The conservatives will have the first opportunity in forming a government. Finland’s system of proportional representation requires a coalition to muster more than 100 seats in the 200-seat parliament to run the country.