Marcus Siepen (born 8 September 1968) is a German guitarist, best known as the rhythm guitarist and backing vocalist of power metal band Blind Guardian. For the biggest percentage of Blind Guardian's songs, particularly in more recent years, he has almost strictly played rhythm guitar, with most lead and solo work being performed by André Olbrich. He has also been part of the band Sinbreed and live member of Demons & Wizards.
