The six-time All-Star said his family finalized the adoption of identical twin girls this week. The babies were born prematurely and Posey said after consultations with his wife and doctor he decided to opt out of the season.
Posey had missed three San Francisco practices while dealing with a personal issue.
The 2012 NL MVP and three-time World Series champion joined Dodgers pitcher David Price, Washington first baseman Ryan Zimmerman, Colorado's Ian Desmond, Arizona pitcher Mike Leake and other in deciding not to take part in the 60-game season this year.
The 33-year-old Posey said it was difficult to miss an entire season but that this was the best decision for his family.
"These babies, being as fragile as they are for the next four months minimum, this wasn't ultimately that difficult a decision for me," he said.
Posey hit .455 in exhibition games this year before spring training was halted March 12 because of the virus outbreak.