Tarence Kinsey, who hit a 5-foot runner and the ensuing free throw for a 72-58 lead with 4:08 remaining, joined Josh Gonner in scoring 14 points for South Carolina, which hit 12-of-21 shots from beyond the 3-point arc.
Since losing leading scorer Jaime Lloreda to an injured left foot for the remainder of the season in a Feb. 18 loss to Auburn, LSU (18-10) has dropped seven of eight. LSU had a 15-point lead midway through the first half, but the Tigers couldn't contain South Carolina in the paint.
The Gamecocks had lost three of four before beating Arkansas 91-81 on Thursday, but Boynton scored all five of his field goals on 3-pointers against the Razorbacks, too.
LSU had reached the semifinals in three of the last four years, and the Tigers had won their first SEC tournament game in the same span.
Carlos Powell led South Carolina with seven rebounds and five assists.
Renaldo Balkman had 11 points to lead the Gamecocks in outscoring LSU 24-13 from the bench.
LSU shot just 32 percent (8-of-25) in the second half as Gonner gave South Carolina its first double-digit lead with a 3-pointer from the right side of the key with 8:35 remaining. The Tigers were just 3-of-15 on 3-point attempts in the second half.
The Gamecocks improved to 16-1 when scoring 70 or more points. South Carolina trailed 40-39 at halftime, but the Gamecocks went to 2-13 when trailing at intermission.