Won praise for her views on women's rights, climate change and diversity, but opinion polls in October showed her popularity waning amid a slowing economy and her party's handling of sexual assault complaints.
She was raised by same-sex parents and told a Finnish magazine how her experience influenced her political career.
Marin had a chance to demonstrate her leadership skills earlier this year, when she stood in for Rinne while he was recovering from a serious illness. The Social Democrats and its main government ally the Center Party were each backed by just over 10% of the population in the latest survey by YLE.
Who else is leading Finland? .
The outgoing Prime Minister, former union leader Antti Rinne, was pushed out of office last week after losing a vote of no confidence in his leadership.
Sanna Marin, 34, from the dominant Social Democrats, was chosen by her party as the sole candidate for prime minister on Sunday. "It will soon be possible to continue our cooperation for a fair and lasting future", Ms Andersson tweeted. As world leaders under 50.
The average age of a member of parliament in Australia at the last election was 51.
The new government, consisting of 12 female and 7 male ministers according to media reports, will be nominated on Tuesday.
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.In a 2015 interview, Ms Marin said she felt "invisible" as a child, because she could not speak openly about her family setup. He is expected to remain the leader of the Social Democrats. Among the five leaders, four, including Marin, are under 35.
She will also be the youngest head of government in Finland's history. When she takes the reins of the country, most likely on Tuesday, she will become the world's youngest sitting head of government. Finland's Sanna Marin is set to be sworn in as the country's prime minister this week. Antti Lindtman, head of the party's parliamentary group, was runner-up.
The coalition will have a comfortable majority of 117 seats in the 200-seat Eduskunta, or Parliament.
The newly appointed prime minister will take over charge amid a three-day strike that may stop production at a few of the country's largest firms from Monday.
Now the frontrunner to replace Rinne, Marin has previously held the position of vice chairwoman for her party and most recently served as the minister for transport and communications, in addition to her role as deputy leader.
