Posey added that he and his wife Kristen are adopting twin baby girls who were born prematurely and will remain in NICU for some time, according to Andrew Baggarly of The Athletic.
The 2012 NL MVP had been weighing his options regarding the shortened schedule amid the coronavirus pandemic. On Friday, he said the decision wasn't hard.
"This ultimately wasn't that difficult (of a) decision for me," Posey said, according to Hannah Keyser of Yahoo! Sports. "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."
"If these babies hadn't been born right now and weren't premature I'd probably be playing," he added, according to Keyser.
Manager Gabe Kapler and general manager Farhan Zaidi expressed their support for Posey's decision.
"I think Buster is making an incredibly thoughtful decision for him and his family and the one that makes the most sense," Kapler said, according to Alex Pavlovic of NBC Sports Bay Area.
The 33-year-old is the latest MLB player to sit out the upcoming season, following Los Angeles Dodgers left-hander David Price, Washington Nationals first baseman Ryan Zimmerman, and several others.
Posey remains under contract for 2021 when he's scheduled to earn $21.4 million. He holds a $22-million club option with a $3-million buyout for 2022.
The six-time All-Star has been a fixture in the Giants' lineup since his first full season in 2010, becoming a key contributor to three championship teams over the last decade. For his career, Posey owns a .302/.370/.456 batting line with 140 home runs and 673 RBIs over 1,258 games.