Sanna Marin, aged 34, will become the world's youngest prime minister as head of a women-led coalition government in Finland.
The transportation minister was picked by her Social Democratic party to take charge after PM Antti Rinne quit. She is expected to be sworn in this week.
She will lead a centre-left coalition of five parties all headed by women.
Mr Rinne stepped down after losing the confidence of a coalition member over his handling of a postal strike.
When she takes office, Ms Marin will be the world's youngest sitting prime minister. New Zealand PM Jacinda Ardern is 39, while Ukrainian premier Oleksiy Honcharuk is 35.
"We have a lot of work to do to rebuild trust," Ms Marin, who won the vote by a narrow margin, told reporters.
She brushed away questions about her age, saying: "I have never thought about my age or gender, I think of the reasons I got into politics and those things for which we have won the trust of the electorate."
Ms Marin will be the third-ever female prime minister in the Nordic country. The Social Democrats emerged as the largest party in elections held in April, and so can appoint the prime minister who leads the coalition government.
According to Finnish broadcaster YLE, Sanne Marin was raised by a single mother and was the first person in her family to go to university.
Finland currently holds the European Union's rotating presidency.
