There's a twist in the Robert Mathis arrest ... his blood alcohol level was BELOW the legal limit, law enforcement sources tell TMZ Sports.
When the ex-Indianapolis Colts star was pulled over at 12:09 AM on Tuesday, we're told he blew a .052 -- well below the legal limit, .08.
So, why was he still arrested for Operating While Intoxicated?
We're told cops conducted a field sobriety test -- Mathis didn't do well -- and officers ultimately determined Mathis was impaired and unable to drive.
FYI, the law allows officers to arrest someone for OWI based on other tests besides B.