Vancouver police said the cause of death was not immediately apparent, but they ruled out foul play.
Medical examiners will conduct an autopsy Monday.
Monteith's body was discovered by staff members at the Fairmont Pacific Rim Hotel after he missed his checkout time, acting Chief Constable Doug LePard told reporters.
Monteith apparently had several people over to his room at one point Friday night, but LePard said Monteith, who had checked in July 6, was seen on hotel surveillance video returning to his room in the early morning hours by himself.
"We are so saddened to confirm that the reports on the death of Cory Monteith are accurate," his rep said in a statement. "We are in shock and mourning this tragic loss."
One of his castmates on GLEE, Mark Salling, tweeted a simple "no" after police held their news conference.
Harry Shum Jr added, "At this time, please send nothing but love and light. This tragic news still doesn't seem real to me. I love you Cory."
Dot-Marie Jones, who plays the football coach at the fictional William McKinley High tweeted: "I have no words! My heart is broken. Cory was not only a hell of a friend, he was one amazing man that I will hold close to my heart forever.
"I am blessed to have worked with him and love him so much! My heart is with his family and our whole GLEE family! I love you all!"
Monteith spent time in rehab this year, checking into a drug addiction treatment facility in late March.
He had been frank about his struggles with substance abuse, telling Parade magazine in 2011 that he began using drugs at 13, and by 19 went into rehab after his mother and friends intervened.
On GLEE, Monteith played the dim quarterback of the football team at the Ohio high school who is forced to join the glee club. After graduation, he comes back to town and helps direct a musical at the school.
In 2011, he won a Teen Choice Award for top actor in a comedy. The show's cast won a Screen Actors Guild Award for an ensemble in a comedy the previous year.
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