The actor's body was found in a Vancouver hotel just weeks after leaving rehab.
The Canadian actor's body was found just after noon at the Fairmont Pacific Rim Hotel, Vancouver police confirmed at a news conference Saturday night. They did not give a cause of death, but foul play was not suspected.
Monteith had recently finished a stint in rehab for substance abuse, after previously seeking help for addiction at age 19.
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"On behalf of the Vancouver Police, I want to pass on our condolences to the family, friends, castmates and millions of fans of Mr. Monteith," acting chief Doug LePard said. "As was the case in countless homes, I watched 'Glee' regularly with my daughters, and I know there will be shock and sadness in many households with the news of his tragic death."
As Finn Hudson on "Glee," Monteith helped make it cool again to sing on scripted shows. He served as a surrogate for those in the audience who weren't sure about embracing music -- and delivered jawdropping renditions of songs like U2's "One" and Michael Jackson's "Man in the Mirror."
"We are deeply saddened by this tragic news. Cory was an exceptional talent and an even more exceptional person," Fox said in a statement on behalf of the network, the show's producers and 20th Century Television. "He was a true joy to work with and we will all miss him tremendously. Our thoughts and prayers are with his family and loved ones."
Monteith had seemed to have turned a corner recently, emerging from rehab and appearing last month with longtime girfriend and "Glee" co-star Lea Michele.
Also read: Hollywood Reacts to 'Glee' Star Cory Monteith's Tragic Death
Police responded within minutes of receiving a call from the BC Ambulance Service regarding a sudden death in a room on the 21st floor of the Fairmont Pacific Rim Hotel in downtown Vancouver.
Upon arriving, paramedics advised the officers that Monteith was dead. Coroner's staff was also on hand.
Officials said no cause of death was apparent, but that Monteith had been dead for several hours before his body was discovered.
According to the police statement, Monteith checked in last Saturday and was due to check out Saturday. Monteith had guests Friday night, but video and key entry records show him returning by himself in the early morning, and police believe he was alone when he died.
Hotel staff discovered his body after he missed check-out.
An autopsy is scheduled for Monday. The Coroner will be determining the next steps with respect to establishing cause of death, but all indications are that there was no foul play.
Born in Calgary on May 11, 1982, Monteith began his acting career in Vancouver.
An original "Glee" castmember, Monteith played Finn Hudson since the Fox series debuted in 2009. He also stars in the upcoming independent film, "All the Wrong Reasons."
The actor admitted himself into a facility for substance addiction earlier this year and completed the treatment program in April.
Monteith leaves behind his parents Joe Monteith and Ann McGregor, who raised him as a single mother, and a brother, Shaun.