Aldon Smith Arrested Again ... Allegedly Violated Bail Condition

Aldon Smith was arrested again and is behind bars after cops in San Francisco say the ex-NFL star violated a condition of his bail ... TMZ Sports has learned.

Smith was booked into the San Francisco County Jail around 5 PM Friday night. He is still in custody. Jail records indicate his bond is $500K.

FYI -- the former Raiders star was in court on Thursday after allegedly violating a protective order put in place following his arrest for domestic violence. The judge ordered Smith to wear a GPS and SCRAM device -- which measures alcohol in your sweat.

According to officials at the San Francisco Sheriff's Department, Smith allegedly violated a condition of his electronic monitoring ... and that's…