|Birth name||Marcus Siepen|
|Born||September 8, 1968|
|Genres||Power metal, speed metal, heavy metal|
|Labels||No Remorse, Virgin, Century Media, Nuclear Blast|
|Associated acts||Blind Guardian, Sinbreed|
Marcus Siepen (born September 8, 1968 in Krefeld, Germany) is a German guitarist and backing vocalist, most known for being the rhythm guitarist 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.
Siepen is divorced and has a son from his first marriage. He has since remarried. Growing up in a Roman Catholic family, Siepen claims to not be religious, and that he doesn't believe in God. He views Jesus Christ as a human philosopher, "a very charismatic person with a great view of how people should live together." 
Since 1995, Siepen has been an endorser of Mesa/Boogie amplifiers, using various Triple Rectifier heads and multiple preamps.
As of 2010, his main rack consisted of a 3-channel Mesa Boogie Triple Rectifier "Recto Reborn" head, with a Fractal Audio Axe-FX II providing effects. He uses 2 Sennheiser wireless units, one as a backup, and his old 2-channel Rectifier as a backup. When doing overseas shows, he solely uses an Axe-FX II. Previous versions of his rack featured the 2-channel Triple Rectifier as his main amplifier, a Rocktron Intelliflex instead of the Fractal for effects, and either a Mesa Boogie TriAxis preamp or Rectifier Recording Preamp instead of his 2-channel Triple Rectifier as a backup.
For guitars, Siepen has made most use of Gibson and ESP Guitars, with Gibson being his preferred brand nowadays. Up until the late '90s, Siepen mostly used ESP guitars with Blind Guardian co-guitarist André Olbrich, both guitarists making use of custom-made Horizon and Mirage guitars, although Siepen has been seen using other shapes, such as a custom-built model based on the Jackson Randy Rhoads. He can sometimes be seen with a custom ESP Eclipse, but he does not use this model often. His current main guitars, since then, have been several Gibsons, mostly Les Paul models, his main two being a Les Paul Custom and a Les Paul Standard Premium Plus. Most of his guitars, including the previously two mentioned models, feature dual EMG 81 pickups, Suhr Doug Aldrich pickups, and TonePros hardware.