Marko Tapani "Marco" Hietala (born 14 January 1966) is a Finnish heavy metal vocalist, bassist and songwriter. Internationally, best known as the former bassist, male vocalist and secondary composer to Tuomas Holopainen, of the symphonic metal band Nightwish. He was also the vocalist and bassist as well as composer and lyricist for the heavy metal band Tarot.
He was also a member of the supergroup Northern Kings, and portrayed one of the main characters in Ayreon's 2013 album The Theory of Everything.