Catcher Derek Norris lost track of the ball as it fell to the grass, then fielded it and threw to second. Belt waited to see whether the ball would fall in, then stumbled as he approached the bag, and Alexei Ramirez, the Padres shortstop, dropped the throw from Norris.
Belt overslid the bag, however, and was tagged out as he lay in the infield dirt.
X-rays came back negative, and Belt said after the game that he could play Friday when the Giants open a three-game series at Colorado.
"It didn't look great," Belt told reporters. "But I feel like unless something happens [on Thursday] and I wake up and it's swollen, I feel like I'll be able to play Friday."
The Giants beat the Padres, 4-3 in 10 innings, fortheir 13th win in 14 games.
Information from The Associated Press was used in this report.
Giants walk-off for fifth straight win
Brandon Crawford's single to deep center in the 10th lifts the Giants to a 4-3 win over the Padres, lifting San Francisco to its fifth straight win.