Powell scored six of the Gamecocks 10 points in the extra period, including four free throws down the stretch. The senior, triple-teamed and falling backward, also hit an 8-foot bank shot with 18.8 seconds left to tie the game at 66 in regulation.
The game mattered little in the Southeastern Conference standings. South Carolina (15-12, 7-9 SEC) finished fourth in the SEC East and will meet the Rebels, who finished fifth in the West, again Thursday in the opening game of the league tournament.
Marvin Moore led Ole Miss (13-16, 4-12) with 16 points. Kendrick Fox added 14 and Todd Abernethy added 13.
Tre Kelley and Tarence Kinsey each added 13 points for the Gamecocks.
The Rebels used a 24-7 first half run to take a 39-29 halftime lead. Ole Miss hit eight of 11 field goals, including four 3-pointers - two by Fox, who had four 3s in the first half.
But Powell scored South Carolina's first six points of the second half and Rocky Trice added a 3-pointer from the corner as the Gamecocks scored nine straight points to cut the deficit to 41-38 with 15:16 left.
After the Rebels pushed their lead back out to seven, Kelley and Kinsey hit back-to-back 3-pointers and then Kinsey scored on a three-point play to give South Carolina a 54-53 lead with just under seven minutes left.
Mississippi battled back to lead 60-56 with a little over four minutes to go. But Kelley swished a 3-pointer from the wing to tie it at 64-64 for the Gamecocks with less than two minutes left.
South Carolina Slams Mississippi, 76-70
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