Posey told reporters Friday that his babies -- Ada and Livvi -- were born about eight weeks prematurely last week and will have weakened immune systems over the next few months. Posey said he and his wife, Kristen, finalized the adoption Thursday.
"In the current state that we are right now and these babies being as fragile as they are for the next four months, at minimum, this ultimately wasn't that difficult of a decision for me," Posey said. "From a baseball standpoint, it was a tough decision. From a family standpoint and feeling like I'm making a decision to protect our children, I think it was relatively easy."
Posey -- a six-time All-Star selection, three-time World Series champion and 2012 NL MVP -- becomes one of the most high-profile players to sit out of the season. He joined other notable players to opt out, including Los Angeles Dodgers pitcher David Price, Washington Nationals infielder Ryan Zimmerman and Colorado Rockies outfielder Ian Desmond.
Giants president of baseball operations Farhan Zaidi and manager Gabe Kapler said they supported Posey's decision and wished his family the best.
"I think Buster is making an incredible and thoughtful decision for him and his family and the one that makes the most sense," Kapler told reporters Friday. "I'm actually proud to be here on this call listening and supporting today."
Posey was scheduled to earn a base salary of about $8 million this season. Under the terms of an agreement between the league and the players' union, the Giants aren't obligated to pay his salary.
Posey, 33, has battled numerous injuries over the past two seasons. He had a career-low .257 batting average last season and finished with seven home runs and 38 RBIs in 405 at-bats.
MLB is set to begin its 2020 season July 23. The league scheduled two games for opening day, including the Giants visiting the Dodgers.