"It is dangerous for me and the people around me if I continue."
writes the bassist on Twitter.
54-year-old Hietala also points a boot at those in power in the music industry, whom he accuses of "sucking every drop out of the artists and then plundering the profits themselves".
Marko Hietala joined Nightwish, Finland's largest metal band outside the country's borders, in 2001.
The announcement of the defection comes two weeks after the compatriots Children of Bodom's frontman Alexi Laiho died as a result of what his colleagues in the band called "long-term health problems".