A source told ESPN that Reid is working with attorney Mark Geragos, who also represented his former teammate, Colin Kaepernick, in the quarterback's collusion grievance against the league.
Reid's grievance is a system arbitrator case that will be adjudicated under the terms of the NFL's collective bargaining agreement. The claims in Reid's grievance are similar to those of Kaepernick's, the source told ESPN.
"Our union is aware that Eric Reid and his legal representatives filed a collusion claim, which will be heard through the arbitration process as spelled out in our collective bargaining agreement," the NFL Players Association said in a statement Wednesday. "Our union supports Eric and we are considering other legal options to pursue."
Reid, 26, remains unsigned after becoming a free agent in March when his rookie contract with the 49ers expired. He was one of the first players to join Kaepernick two years ago in kneeling during the national anthem to protest police brutality and racial inequality.
Reid has since continued his protests during the anthem. He tweeted at the beginning of the offseason that he wasn't receiving interest from teams because of his actions during the anthem, but multiple executives, including Niners GM John Lynch, have cited a "slow" market at safety.
ProFootballTalk reported last month that Cincinnati Bengals owner Mike Brown asked Reid during a recent meeting whether he planned to continue kneeling during the anthem. Reid, who has said this offseason that he planned to stop protesting, did not end up getting a contract offer from the Bengals.
The Bengals are the only team that has hosted Reid for a free-agent visit so far this offseason.
Reid was a first-round pick by the 49ers in 2013 and made the Pro Bowl that year as a rookie. His four-year, $8.4 million contract expired at the end of the season.
Reid also originally was part of a players coalition that met with NFL owners last season to discuss the issues at the heart of the protests. But Reid and others later split with that group due to differences of opinion on the group's priorities. Reid and others believed Kaepernick needed to be a bigger part of the movement and the discussions with the owners going forward.
Schefter: Reid grievance to have 'professional consequences'
Adam Schefter says by Eric Reid filing a collusion grievance against the NFL, his chances of playing again go down dramatically.