Police said the cause of death was not immediately apparent, but they ruled out foul play.
Medical examiners will conduct an autopsy on Monday.
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.
“We are in shock and mourning this tragic loss,” his publicist, Melissa Kates, said in an e-mail.
Monteith spent time in rehab early this year, checking into a drug addiction treatment facility in April.
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 prior year.