Posey, 22, will make his Triple-A debut on Thursday against Colorado Springs when the Grizzlies return from the Triple-A All-Star break.
The Florida State product was San Francisco's first-round pick in the 2008 Draft and has excelled this season with Class A Advanced San Jose, where he was hitting .326 with 13 homers and 58 RBIs in 80 games for the Giants. He was 15-for-37 (.405) with 10 RBIs and 12 runs in his last 10 games for San Jose.
Posey had been rumored to skip Double-A Connecticut and join Fresno ever since the Giants promoted three other top prospects to the Defenders, pitchers Madison Bumgarner and Tim Alderson and shortstop Brandon Crawford.
Posey had missed some games in late June after getting hit in the head with a pitch, but has shown no ill effects since his return, batting .357 in a dozen games this month.
The Georgia native played at two levels in the Giants' system last summer. He's currently ranked as the No. 19 prospect in the Minors by MLB.com.
Danny Wild is an editor for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.