The actor "has voluntarily admitted himself to a treatment facility for substance addiction," his rep tells PEOPLE. "He graciously asks for your respect and privacy as he takes the necessary steps towards recovery."
Monteith, 30, has been open about his past struggles with substance abuse. He admitted in a 2011 interview that he had previously entered rehab when he was 19.
At the time, the actor said he hoped being honest about his struggle with substance addiction would help others.
"I don't want kids to think it's okay to drop out of school and get high, and they'll be famous actors, too," he said in 2011. "But for those people who might give up: Get real about what you want and go after it."