Tejada's go-ahead, two-run shot came off Dodgers pitcher Ted Lilly. It stood as the winning margin, helping to move the Padres back into first place by a half-game in the National League West.
"I'm pretty happy in this situation," said Tejada, who became the 22nd active player to reach the 300-home run plateau and 129th player overall to reach the milestone. "I'm not a home run hitter. I just go and swing and try to hit the ball hard. I'm more happy not with the 300, but that the 300th got the win. This is a great night for the Padres."
The home run was the eighth for Tejada since he joined the Padres on July 29 in a trade with the Orioles. He hit seven home runs in 97 games with the Orioles.
Tejada has hit his eight home runs with the Padres in 48 games, including his first home run with San Diego on Aug. 3 against Lilly at Dodger Stadium.