Dee Bost added 14 points and 10 rebounds for the Bulldogs (21-12), who won their fourth straight and might have eliminated the Gamecocks from consideration for the NCAA tournament.
South Carolina (21-9) has lost three of four down the stretch, a skid that could offset the team's strong RPI (No. 49).
Mississippi State, meanwhile, advanced to play No. 20 LSU, the league's regular-season champion, in one of Saturday's semifinals. The Bulldogs probably need one more win to secure a spot in the NCAA field.
The Gamecocks felt like they let this one get away. They had a double-digit lead early and a four-point advantage at halftime.
But they were outplayed in the second half. Stewart did a big chunk of the damage, hitting consecutive 3s that put Mississippi State ahead for good.
His second one gave the Bulldogs a 58-55 advantage with about 7 minutes to play. Bost hit another 3 from behind the arc a few minutes later. Stewart added two free throws, and Ravern Johnson might have done in South Carolina with a long jumper from the wing that made it 70-61 with 1:56 to play.
Jarvis Varnado, Mississippi State's inside force who helps create all those outside shots, finished with 12 points and nine boards.
Zam Fredrick led the Gamecocks with 20 points and Dominique Archie added 16.
Devan Downey was 5 of 20, including 1 of 7 from 3-point range, and finished with 11 points. It was an off game for the team's leading scorer, who also was called for an intentional foul in the closing seconds.
The Bulldogs hope to avoid another start like they had against South Carolina. Mississippi State missed 10 of its first 12 shots and trailed 14-4 in the opening minutes.
Brian Johnson and Varnado got the team on track.
Johnson had two putbacks, a layup and free throw, scoring seven consecutive points to end the rough start. Varnado added two dunks, and all of a sudden, Mississippi State had whittled South Carolina's 10-point lead to 22-18.
The Gamecocks pulled out to another double-digit lead, but the Bulldogs - who have taken more 3-pointers than any other SEC team - got hot from long range.
Ravern Johnson hit a 3-pointer, Kodi Augustus followed a few seconds later with his first of two in the opening 20 minutes and Mississippi State was back in it heading into halftime.