Harry Shum Jr., who portrays dancer Mike Chang on the show, expressed disbelief on Twitter.
"This tragic news still doesn't seem real to me," said Shum. "I love you, Cory."
John Stamos, who appeared as a guest star on several episodes of "Glee" tweeted: "Heartbreaking. RIP Cory. We talked about how lucky he felt to be alive-and sober. We talked about playing drums. Glad i knew you Cory." Monteith played drums in a California-based indie rock band Bonnie Dune.
Matthew Morrison, who plays glee club coach Will Schuester, issued condolences before performing at a Sunday afternoon show at the cabaret 54 Below in New York, according to the New York Times. The newspaper said he changed his opening number to sing an a cappella rendition of "What I Did for Love" from the musical, "A Chorus Line," which was featured during the second season of "Glee."
"You guys came to see a show, I came to perform a show, so that's what we'll do," he said, according to the newspaper. "And we'll do it in Cory's honor."
The Times said Morrison went on with a set of show tunes and jazz standards, dancing on stage and quipping with the audience about the infamous drug-den history of the space under the 1970's club, Studio 54, where the performance took place.
Fox and the producers of "Glee," including 20th Century Fox Television, called Monteith an exceptional performer "and an even more exceptional person. He was a true joy to work with and we will all miss him tremendously."
"We are in shock and mourning this tragic loss," his representatives at Viewpoint Public Relations in Los Angeles said in a statement.
Monteith, who turned 31 on May 11, starred in "Glee" as a high school football player who puts his status and popularity at risk to join the glee club and its outcast members.
The show, with its pop music-based song-and-dance numbers and high-profile guest stars including Gwyneth Paltrow, became an immediate hit and made stars of its relatively unknown cast.
The series, which debuted in 2009, is in its fourth season.
On his Twitter account, Monteith described himself as "tall, awkward, canadian, actor, drummer, person."
He had recently shot a Canadian film called "All the Wrong Reasons," slated to be released later this year with actors Kevin Zegers, Karine Vanasse and Emily Hampshire.
In a 2010 interview with The Associated Press, Monteith was upbeat about life. He said that if "Glee" were to be canceled he would be OK.
"I've never been afraid of working," he said. "I've never been afraid of auditioning for jobs. Obviously, I've never been afraid of anonymity. I was happy (before 'Glee'). I'm happy now. I guess I'm well adjusted."
Monteith was among the "Glee" actors who remained series regulars as their characters graduated high school and moved on to other adventures. However, in real life Montieth dropped out of high school in Vancouver after his parents divorced.
According to his biography on Fox's website, Monteith was born in Calgary, Alberta, and moved to Vancouver Island as a child. Before turning to acting, he held a variety of jobs including Wal-Mart greeter, school bus driver, roofer and cab driver.
"Thanks for always being kind Cory. You came a long way from hanging on the beaches in Vancouver with the gang pre-Glee," tweeted Gerard Funk, an actor from Vancouver who joined the "Glee" cast last year.