Bryant led Mississippi State with 16 points, while Arnett Moultrie added 15 points, including three key free throws to send the game into overtime.
With the loss, South Carolina (10-19, 2-13) is guaranteed to finish last in the SEC for the second time in six seasons. Bruce Ellington led the Gamecocks with 17 points, but his high-arching short jumper to win the game at the end of regulation was just short.
The Bulldogs needed this win desperately. They were ranked No. 20 in the nation at the beginning of February, but lost five in a row, including defeats against Georgia and Auburn, two of the SEC's worst teams.
And it looked bleak late after a 3-pointer from Ellington and a nice pass from the point guard to Damontre Harris for a dunk put South Carolina up 58-55 with a minute to go in regulation. But Moultrie took the ball inside, making a lay-up as he was fouled and burying the free throw to tie it.
After Anthony Gill hit two free throws for South Carolina on the next possession, Moultrie again pounded the ball inside, got fouled and sank both free throws to tie it at 60 with 11.2 seconds to go.
Ellington missed the shot to win it in regulation, but buried a 3-pointer and a driving lay-up. But Bryant had his own jumper and Bost hit a 3 to tie it at 65 with 1:36 to go in overtime. Renaldo Sidney hit a shot, and Gill answered with a putback to tie it at 67. As the clock wound down, Ellington stripped Bost, but his foot was out of bounds, setting up the winning shot with 1.7 second to go.
Mississippi State finishes with Auburn this weekend as the Bulldogs try to make the NCAA tournament for the first time in four seasons. Bost added 13 points and Moultrie had 12 rebounds, while Sidney added 12 points and 14 rebounds for Mississippi State.
The tailspin continues for South Carolina, which will have to beat Georgia on Saturday to avoid finishing with its worst record ever in SEC play. The Gamecocks have lost 24 of their last 28 SEC games and have won fewer league games in each of coach Darrin Horn's four seasons.
Harris added 14 points and had 11 rebounds, while senior Malik Cooke added 11 points in his final home game.