People.com said no cause of death has been reported, but CPR was attempted and unsuccessful.
TMZ said Guist may have fallen from his boat and witnesses said he appeared to suffer from a seizure.
"We are extremely saddened to report that our friend and beloved member of the 'Swamp People' family, Mitchell Guist, has passed away earlier today," The History Channel said in a statement Monday. "Mitchell passed on the swamp, doing what he loved. We appreciate your respect for the Guist family's privacy and hope you join us in sending our thoughts and prayers to his brother, Glenn, and the rest of the Guist family.
The network said Mitchell Guist's birth date was May 18, 1964.
He and his brother, Glenn, "were born and raised on the bayou and live entirely off the land," The History Channel said on its Web site in a biography for the brothers.
"These brothers are always up to something, whether it's hunting for their dinner or making use of everything nature has to offer. This season, the boys are hoping to repair their homes with cypress wood from the swamp and maybe even catch a gator. One thing is for sure: Their adventures always lead to interesting places," the biography said.