The actor, according to his representative, "has voluntarily admitted himself to a treatment facility for substance addiction. He graciously asks for your respect and privacy as he takes the necessary steps towards recovery."
Monteith, 30, had previously entered rehab at 19 years old.
Monteith's girlfriend, Spring Awakening actress and "Glee" co-star Lea Michele, told People, "I love and support Cory and will stand by him through this. I am grateful and proud he made this decision."
"Glee" is currently in production on its fourth season, with two episodes remaining to shoot.
"Glee" co-creator Ryan Murphy tweeted, "I am so proud of Cory Monteith and love and support him 100%."
Dot-Marie Jones, who plays coach Shannon Beiste on "Glee," tweeted, "U NEVER KNOW WHAT SOMEONE IS DEALING WITH IN THEIR PERSONAL LIFE, DON'T PASS JUDGMENT AND LOVE AND SUPPORT IT'S WHAT I BELIEVE IT WHAT I DO."