Drew Lock's 61-yard touchdown pass to Jason Reese with 13:07 remaining in the first half gave Missouri (1-1, 0-1 SEC) its 10-0 lead, but Samuel changed the game's momentum by returning the ensuing kickoff 97 yards for a TD, his second kickoff return for a score in as many weeks.
On the next play from scrimmage, South Carolina's Jamyest Williams intercepted a Lock pass at the Tigers' 25-yard line. Samuel ran 25 yards for a touchdown on first down, giving the Gamecocks (2-0, 1-0) a 14-10 lead with 12:37 left in the half.
Jake Bentley's 40-yard touchdown pass to tight end Hayden Hurst with 8:41 to play in the third quarter extended the South Carolina advantage to 21-10.
Tucker McCann's 22-yard field goal pulled Missouri to within 21-13 with 6:31 to go in the third quarter, but Gamecocks' kicker Alexander Woznick's 32-yard field goal with 18 seconds left in the third period re-established an 11-point lead at 24-13.
After South Carolina recovered a fumbled punt, Hurst scored on a 2-yard run with 10:25 left in the fourth quarter to give the Gamecocks a 31-13 advantage.
Lock, who threw an SEC-record-tying seven touchdown passes in Missouri's season-opening victory, was limited to one TD pass and was intercepted twice. South Carolina scored 14 points as the result of Missouri turnovers.
Bentley finished 18 of 28 for 187 yards, one touchdown and no interceptions.
Missouri's Damarea Crockett was the game's leading rusher with 97 yards.
South Carolina is 2-0 for the first time since 2012.