A hotel employee found Gray, 38, dead in a room at the TownePlace Suites in Urbandale, a suburb of Des Moines, police said in a statement. An autopsy was planned for Tuesday.
"Paul Gray was an awesome person on and off stage. He will be missed by many including myself," Jacoby Shaddix, lead singer of metal band Papa Roach, said in a statement late Monday. "His spirit will live on through the killer music he wrote."
Gray had been staying at the hotel for "a couple days," Urbandale Sgt. Dave Disney said, declining further comment. Gray lived in the nearby suburb of Johnston.
Amy Sciarretto, a publicist at Slipknot's record company, Roadrunner Records, confirmed Gray's death but declined further comment.
Known for its grotesque masks, trashing sound and aggressive, dark lyrics, Slipknot released its self-titled debut in 1999, and it sold about 2 million copies. Most of the band's members grew up in the Des Moines area.
"It's a devastating loss. Paul was a wonderful human being," said Andy Hall, music director of Des Moines rock station Lazer 103.3 who said he'd known Gray for 10 years. Hall said Gray was a talented bass player and one of the friendliest, most caring people he knew.
"This is a big blow, not only to the community of Des Moines but fans of metal at large, worldwide," Hall said, noting that his station planned to broadcast an hour-long tribute to Gray on Monday night.
Slipknot emerged in the mid-1990s with an aggressive mix of heavy metal and a vocal style that included growling, rapping and singing. The band has been known for extreme behavior during live performances, including urinating and vomiting on stage, according to biographies.
The band won a Grammy in 2006 for best metal performance for the song "Before I Forget," and concert industry trade publication Pollstar ranked Slipknot 18th in its Top 20 Concert Tours list in 2009.
In 2003, Gray acknowledged that he was on drugs when his red 2001 Porsche collided with another car that year in Des Moines. No one was seriously injured. Under a plea agreement, prosecutors dropped charges of possession of marijuana, cocaine and syringes.
Media reports at the time noted that court records included a handwritten note from Dr. Joe Takamine that described discussions with Gray that were "very frank and open about his sporadic use of various drugs and of the long periods of abstinence in between."
Tom Ramirez, a drummer from Des Moines who became friends with Gray in high school, said he ran into Gray at a concert a few months ago and thought Gray "looked great." Ramirez remembered Gray as someone who always made time for his fans and old friends in Des Moines.
"He was always accepting and he wasn't stuck up. He was a people person. He knew his friends and who his friends were," Ramirez said. "He didn't forget the little people back here."
Slipknot remains one of the most popular metal bands and can still fill arenas, said David Gehlke, editor in chief of blistering.com, a heavy-metal and rock website.
The band is on a yearlong hiatus, and Gray planned to play with Hail, an all-star metal band that includes the former lead singer of Judas Priest and covers songs by that band, Motorhead and Iron Maiden, Gehlke said.
"This is going to be quite the blow to Slipknot and their fan base," Gehlke said during a telephone interview from Pittsburgh.
Gehlke noted the deaths of heavy metal singer Ronnie James Dio, who performed with several bands including Black Sabbath, last week and singer-bassist Peter Steel last month.
"This is just a big surprise for a lot of us and it's a shame too," Gehlke said. "We just had Dio pass away, Peter Steel from Type O Negative - three pretty significant blows to heavy metal community."