The newest Oakland superstar has worn the same brand of helmet throughout his Hall of Fame-worthy career ... but the NFL announced this year it's banning players from using it.

According to NFL Network's Michael Silver ... Brown tried to skirt the league ban by painting his old helmet, hoping team officials wouldn't notice.

Spoiler alert: They noticed.

Now, Schefter is reporting Brown is SO upset over not being able to use his favorite headgear -- he's threatening to RETIRE if he doesn't get his way!!

Of course, Antonio has been nothing but drama since being traded from the Pittsburgh Steelers to the Raiders this offseason. He's been sued for not paying a trainer, and he's also dealt with foot injuries caused by a freakish cryotherapy mishap.

But, this latest helmet problem seems to have topped everything ... because one anonymous coach told Silver, this is "honestly the most insane thing I have ever heard."

Brown has missed almost every one of the Raiders' training camp practices this summer ... and it's reportedly unknown when -- if ever -- he'll rejoin the team at camp.