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."
People caught up with Monteith's Glee co-star and girlfriend Lea Michele, who had this to say.
"I love and support Cory and will stand by him through this," she tells PEOPLE exclusively. "I am grateful and proud he made this decision."