The seat became vacant after MEP Sampo Terho was elected to the national assembly.
On Monday, Ruohonen-Lerner received an official notification on her place at the European Parliament.
According to Ruohonen-Lerner, the decision to head to the European Parliament was not easy.
"The economic issues have to be in order and the debt brought to control in order to secure the most essential services for the Finns in the future," said Ruohonen-Lerner.
Her place in the parliament will be taken by Leena Meri.
