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.
He had checked into his room July 6.
The actor apparently had several people over to his room at one point Friday night, but LePard said Monteith was seen on hotel surveillance video returning to his room in the early morning hours by himself.
One of his castmates on "Glee," Mark Salling, tweeted a simple "no" after police held their news conference.
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!"
On the show, 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.
Offscreen, he was dating co-star Lea Michele.
"We are in shock and mourning this tragic loss," his publicist, Melissa Kates, said in an e-mail.
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.
Monteith had been on the musical comedy show since it began in 2009.
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.
He was in three projects that are in post-production, according to the Internet Movie Database.
One of them was a movie entitled "All The Wrong Reasons," also starring Kevin Zegers. He tweeted that his heart was broken.
"I've never lost a friend this close. This feels like a mistake," he wrote.