"We are all shocked and saddened by the devastating and untimely loss of Kimbo Slice, a beloved member of the Bellator family," Bellator president Scott Coker said in a statement, calling Slice "a charismatic, larger-than-life personality that transcended the sport."
"Outside of the cage he was a friendly, gentle giant and a devoted family man," Coker said. "His loss leaves us all with extremely heavy hearts, and our thoughts and prayers are with the entire Ferguson family and all of Kimbo's friends, fans, and teammates."
There was no word on the cause of Slice's death.
Slice had been hospitalized earlier Monday in Margate, Florida, for undisclosed reasons, according to Coral Springs police, who had been dispatched to his residence to prevent a potential gathering outside. They said no foul play was suspected.
"We lost our brother today," Slice's longtime manager, Mike Imber, said in a text message to The Associated Press.
Slice, birth name Kevin Ferguson, was a former backyard brawler and internet sensation. A heavyweight at 6-foot-2, 225 pounds, he had a 5-2 professional record with four TKOs.
He was signed to Bellator MMA and scheduled to headline Bellator 158 on July 16 in London against James Thompson.
He last fought at Bellator 149 on Feb. 19 in Houston. He defeated Dhafir Harris, aka Dada5000, in a three-round decision. The result was later changed to a no-contest by the Texas commission, after Slice tested positive for anabolic steroids and an elevated testosterone ratio.
Slice also previously fought for the UFC.
"He carried himself as a true professional during his time in our organization," the promotion said in a statement Monday night. "While he will never be forgotten for his fighting style and transcendent image, Slice will also be remembered for his warm personality and commitment to his family and friends."
Slice was born in the Bahamas on Feb. 8, 1974, but grew up in South Florida. He played middle linebacker at Miami's Palmetto High and showed the potential to play in college before Hurricane Andrew caused Palmetto High's season to be cut short and his scholarship offers vanished. He flunked out of college at Bethune-Cookman University and was homeless for a brief time. He worked as a limo driver, strip-club bouncer and bodyguard before rising to fame through his viral street-fighting videos.
He was not embraced by much of the MMA world as it attempted to go mainstream, with UFC president Dana White famously saying Slice would not last two minutes in the Octagon. However, due in part to his immense popularity, Slice's third professional fight, a fourth-round TKO against Thompson in May 2008, aired on CBS, making it the first MMA fight on prime-time network television.
In 2009, the UFC booked Slice as a contestant on "The Ultimate Fighter" reality series. He ultimately fought for the UFC twice, compiling a 1-1 record, before taking a leave of absence from MMA to compete in professional wrestling.
In 2015, Bellator signed Slice and promoted him in a main event against MMA pioneer Ken Shamrock. Slice won the fight via TKO in the first round, after nearly being submitted by Shamrock in the opening minutes.
Shamrock tweeted about Slice's death Monday night.
The two Bellator events Slice competed in, Bellator 138 and Bellator 149, set new ratings records on Spike TV.
Slice made his professional MMA debut on Nov. 10, 2007, for the now-defunct promotion EliteXC, knocking out Bo Cantrell in just 19 seconds.
He trained out of American Top Team in Coconut Creek, Florida. The team also mourned his passing on Twitter.
For all of his glowering in-cage swagger and outsized fame, Slice was extraordinarily honest about his fighting abilities. He acknowledged being an MMA newcomer with much to learn, never claiming to be anything but a big puncher providing for his family while constantly working to learn the sport's other disciplines.
"The guys who are holding the titles, heavyweight and light heavyweight, these guys are awesome," Slice told the AP in a 2010 interview before his second UFC fight. "I'm really just having happy days in the midst -- being among them, fighting on the undercards, just contributing to the UFC and the sport. That's really what I want to do. I'm not looking ahead to winning a title or anything like that. I'm just enjoying each fight as it comes."
Slice is survived by six children, and he credited his MMA career for allowing him to send them to college. One of his three sons, Kevin Ferguson Jr., made his MMA debut in March.
The Associated Press contributed to this report.
Le Batard: Kimbo Slice fought his way toward a better life
Dan Le Batard discusses Kimbo Slice's deep roots in Miami, how he emerged from the streets to become one of the first viral sports stars by fighting in the streets, and what his legacy is in the sport of mixed martial arts.
Okamoto: Kimbo will be remembered as a fighter through and through
ESPN MMA writer Brett Okamoto looks back at the MMA career of Kimbo Slice, who died Monday at age 42.
Kimbo was more than his persona
Ariel Helwani of MMAFighting.com reacts to the death of Kimbo Slice by saying, "he was one of the sweetest, kindest athletes I've ever come across."
Kimbo's final victory
Go back to June 19, 2015, when Kimbo Slice was able to slide out of a submission attempt from Ken Shamrock to then knock him out seconds later at Bellator 138. The victory turned out to be the final official win of Slice's career.