Kotsay, 36, entered Sunday batting .275 with one home run, six doubles and nine RBI in 55 games for the Padres this year. He has started in left field, right field and first base while going 9-for-31 as a pinch-hitter.
"Mark provides versatility and a very good late-game at-bat," said Padres general manager Josh Byrnes. "He brings invaluable leadership to our club and has become one of the key members of our team."
The California native also played for the Padres from 2001-03. He is a career .278 hitter with 125 home runs and 703 RBI.