Colin Kaepernick has filed a lawsuit against NFL owners for what his lawyer Mark Geragos alleges is their collusion in denying the quarterback employment.
Kaepernick last year started the concept of taking a knee during the national anthem before games as a protest against perceived racial inequality.
Despite being a starter for the San Francisco 49ers, the 29-year-old has not been recruited by an NFL team since becoming a free agent in March and blames his political views for being ignored.
"We can confirm that this morning we filed a grievance under the CBA on behalf of Colin Kaepernick," Geragos said in a statement released on Twitter.
"This was done only after pursuing every possible avenue with all NFL teams and their executives.
"If the NFL (as well as all professional sports leagues) is to remain a meritocracy, then principled and peaceful political protest - which the owners themselves made great theatre imitating weeks ago - should not be punished and athletes should not be denied employment.
"Such a precedent threatens all patriotic Americans and harkens back to our darkest days as a nation. Protecting all athletes from such collusive conduct is what compelled Mr Kaepernick to file his grievance.
"Colin Kaepernick's goal has always been, and remains, to simply be treated fairly by the league he performed at the highest level for and to return to the football playing field."
Kaepernick's act of taking a knee has been emulated by other NFL players, resulting in US president Donald Trump last month calling for the protesters to be fired by their teams, calling them "sons of b******".
Trump also posted a series of tweets in which he insisted "the issue of kneeling has nothing do with race".
Six days ago NFL commissioner Roger Goodell sent a memo to all team owners in which he said "everyone should stand for the National Anthem".
"The current dispute over the National Anthem is threatening to erode the unifying power of our game, and is now dividing us, and our players, from many fans across the country," Goodell said.
The memo came after US vice president Mike Pence walked out of the Indianapolis Colts' game against the San Francisco 49ers after a large number of 49ers players took a knee during the anthem.