Schwartz, 30, signed with the Detroit Lions in free agency only to be released by the team before the start of last season.
"I've known that I've been retired for months, so have others around me. Now y'all know, as well," wrote Schwartz, who played both guard spots and right tackle during his NFL career.
He was originally drafted in the seventh round out of Oregon by the Panthers and played two seasons in Carolina before heading to the Vikings for one year, the Chiefs for a season and then the Giants for two seasons.
Schwartz was a heralded free-agent signing by the Giants two offseasons ago, but injuries severely limited his impact. Toe and ankle problems limited him to just two games in 2014, and he missed the final five games of the 2015 season when ankle injuries flared up again.
He played left guard, right guard and right tackle during his time with the Giants, who released him in February 2016 with two years left on his deal and $500,000 of his 2016 salary already guaranteed.
Schwartz's brother, Mitchell Schwartz, is a right tackle for the Chiefs.
ESPN Lions reporter Michael Rothstein contributed to this report.