The news came just hours after Metso CEO Nico Delvaux announced he was leaving the company.
The Finnish tax authority's decision concerns transactions between Metso Minerals Oy and its foreign subsidiaries.
In a statement Metso said it considered the Finnish tax authority's decision to be unfounded and planned to appeal the decision to the Board of Adjustment of the Finnish tax authority.
However, it will recognise the income tax liability in the full amount of €21 million in its financial statements and results for 2017.
Delvaux has resigned from Metso to take the position of president and CEO of Assa Abloy AB.
"I was offered a once in a lifetime opportunity that I could not turn down," he said.
"I have enjoyed my time at Metso and I'm grateful for the board and all the Metso people for the opportunity to work with them over the past months."
Metso's share price closed yesterday at €28.61.