Rinne resigned earlier this week after coalition member the Centre Party said it had lost confidence in him following his handling of a postal strike.
“We have a lot of work ahead to rebuild trust,” Marin told reporters after winning a narrow vote among the party leadership. Antti Lindtman, head of the party’s parliamentary group, was runner up.
“We have a joint government programme which glues the coalition together,” Marin said.
The coalition, which took office just six months ago, has agreed to continue with its programme after Rinne announced he was stepping down at the demand of the Centre Party.
The timing of the change in leadership is awkward for Finland, which holds the rotating presidency of the European Union until the end of the year, playing a central role in efforts to hammer out a new budget for the bloc. (Reporting by Tarmo Virki Editing by Peter Graff)