In 2012 spring training, Whiteside competed with Stewart for the backup catcher job. However, the Giants ultimately decided to give the role to prospect Héctor Sánchez, a promising hitter who had been expected to start the season with Fresno to work on his defense. Stewart was traded to the Yankees and Whiteside was sent to Fresno. In 60 games (201 at bats) with Fresno, he hit .224 with 45 hits, one home run, and 20 RBI. On July 18, 2012, Whiteside was called up from Fresno when Héctor Sánchez was placed on the disabled list, appearing in six games before being optioned back to Fresno on August 1 when Sánchez returned from the disabled list. He was then recalled on August 26 to be the third-string catcher. Whiteside appeared in 12 games with the Giants in 2012, having one hit (an RBI double) in 11 at bats. He threw out three out of five attempted base stealers. This time, he was left off the playoff roster as the Giants went on to win the World Series in a four-game sweep against the Detroit Tigers.