Julio Urías gave up two one-out singles to put runners on first and second in the bottom of the ninth.
After striking out Brandon Belt, Urías was replaced by Pedro Báez. Posey got ahead in the count and he was able to drive a single to left to score the winning run.
The poor ninth inning should diminish the start by Dodger left-hander Hyun-jin Ryu.
Ryu matched his career high with an eight-inning effort on Wednesday. He allowed a run and four hits. Ryu struck out six and did not walk anyone. For the season, Ryu now has 39 strikeouts and just two walks.
The Giants were able to score a run after their third batter hit in the first inning. A single and double put runners on second and third. A long sacrifice fly by Brandon Belt scored the run that gave the Giants a 1-0 lead. That run was only the second run the Giants had scored in the first inning this season.
That score remained that way until the sixth inning, Kiké Hernández who entered the game hitting .500 against Madison Bumgarner. He was 21-for-42 with four home runs.
Hernández, who had singled earlier, doubled off the wall to put him in scoring position. But he was still there with two outs. Cody Bellinger hit a ground ball down the right-field line. Belt was able to field it but Bellinger was able to beat the throw. Meanwhile, Hernández had been trying to steal third and he was able to go home to score the tying run.
Bumgarner pitched well to Dodgers not named Kiké Hernández. He pitched six innings and gave up one run and four hits. Bumgarner walked one and struck out eight.
Reyes Moronta followed Bumgarner and he was able to work his way out of first and second with no outs. After Ryu struck out attempting to bunt, Moronta was able to strike out Hernández and Seager to end the threat.
The Dodgers would leave eight runners on base and they went 1-for-10 with runners in scoring position.
WP: Will Smith (1-0) 1 IP, 1 hit
LP: Julio Urías (2-2) ⅔IP, 1 run, 2 hits, 1 SO
The Dodgers have their first of five off-days in May tomorrow. On Friday, they will see former teammate Manny Machado and the San Diego Padres for the first time this season. Clayton Kershaw will take the mound against Eric Lauer, first pitch will be at 7:10 p.m.