Moments after Buster Posey, the star catcher who guided the San Francisco Giants through the team’s greatest era with three World Series titles, surprisingly announced his retirement on Thursday, one of his former bosses wanted to be sure.

“Is this a definite, like for sure thing?” Farhan Zaidi, the Giants’ president of baseball operations, said while seated near Posey during a news conference at Oracle Park, the team’s stadium. “I just had to ask.”

Posey was indeed serious — even after enjoying one of the best seasons of his career and leading the Giants to a franchise-record 107 wins and a National League West division title during the regular season. Despite those achievements, Posey, 34, felt it was time to walk away from playing.

“I want to be able to do more stuff from February to November with my family,” he said. “Physically it’s much harder now. And to be honest, it’s hard to enjoy it as much when there’s the physical pain that you’re dealing with on a daily basis.

“I halfway joke with our training staff about being done five years ago. But I don’t know how far of a stretch that’s really off without the work they were doing daily, with a history of an ankle injury and then a hip. It was just getting to the point where things that I was enjoying were not as joyful anymore.”

Playing 12 years in the major leagues takes its toll on the body, but no position is more demanding than catcher. Still, despite the wear and tear and injuries, Posey became one of the most accomplished players of his generation.

The Giants selected Posey, a Georgia native, out of Florida State University with the fifth overall pick in 2008, and he produced a splendid résumé that may one day land him in the National Baseball Hall of Fame.

Posey won the 2010 N.L. Rookie of the Year Award and the 2012 N.L. Most Valuable Player Award in a career in which he won a batting title, was named to seven All-Star teams, received four Silver Slugger awards and won one Gold Glove Award. And, of course, he won World Series titles in 2010, ’12 and ’14. Since the start of the 2010 season, only Mike Trout, the Angels’ three-time winner of the A.L. M.V.P., has recorded more wins above replacement (77.8) than Posey (57.8), according to FanGraphs.

“He not only excelled on the field, but he was a quiet leader that lead by example,” said Giants Chairman Greg Johnson, who noted that he cried after Posey called last week to give him the news. “He has been the consummate professional, both on and off the field.”

The M.V.P., batting title and World Series championship in 2012 may be his most impressive achievements on the field and they came shortly after his career was nearly derailed. During a game in May 2011, Posey was injured in a collision at the plate with the Florida Marlins’ Scott Cousins. A lower leg bone was broken and three ligaments in his ankle were torn. Screws were inserted during surgery. Many worried if he would ever be the same. (The incident eventually led to new rules to protect catchers from such collisions.)

After an excruciating rehabilitation, Posey returned in 2012 to hit .336 with 24 home runs and 103 runs batted in over 148 games and win his second ring. Six years later, he also underwent surgery for a hip injury.

At his news conference on Thursday, Posey declined to detail how much his body had hurt in recent years because he said he didn’t want to make excuses for any down performances. He said going out on his own terms was a factor in his decision, given that so many star players see a steep decline or eventually get pushed out.

Being around his family more was a bigger reason to retire. Posey forfeited his salary and opted out of the 2020 coronavirus-shortened season to be at home after he and his wife, Kristen, adopted twins, who were hospitalized in a neonatal intensive care unit after birth.

Posey said he returned this season because he wanted to prove that he could still play at a high level after having the worst season of his career in 2019 and missing 2020. Rested throughout the season, he produced the second highest on-base plus slugging percentage (.889) of his career and led the surprising Giants in upsetting the defending World Series champion Los Angeles Dodgers, who had won eight consecutive division titles, for the N.L. West crown. (They fell to the 106-win Dodgers in a division series in the playoffs.)

Entering 2021, the final guaranteed season of his nine-year $167 million extension, Posey said he had a feeling it would be his last. And as the season went on, he never wavered. Even after playing well, he wasn’t swayed.

“That’s part of the reason I do feel at peace with my decision, because it would obviously be much harder if I felt otherwise,” said Posey, whose contract had a $22 million club option for 2022.

Posey said he would miss the camaraderie of and competing alongside his teammates and team officials, and seeing fans everyday. But he said he expected to remain involved with the Giants somehow. Johnson said the team was still ironing out a future role for Posey off the field.