Oakland Raiders cornerback Sean Smith -- who signed a $40 million contract last year -- was arrested Thursday and charged with felony assault, TMZ Sports has learned. 

Officials say Smith attacked his sister's boyfriend on July 4 -- beat and stomped on his head in Pasadena in the early hours. 

He's been charged with felony assault by means of force likely to produce great bodily injury and battery with serious bodily injury. 

If convicted, Smith faces up to 7 years in state prison. 

It's not Smith's first brush with the law -- he was convicted of DUI stemming from a 2014 car crash in which he smashed into a light pole. 

We've reached out to the Raiders for comment -- so far, no word back. 

Smith's attorney, Daniel Rosenberg, says Smith maintains his innocence and vows to fight the charges.