Olympic sprinter Fred Kerley is once again in trouble with the law ... TMZ Sports has learned he was arrested on Thursday after he allegedly struck his ex-girlfriend in the face.
According to an arrest report we obtained ... cops were dispatched to Dania Beach, FL around 7:48 PM to investigate Kerley after Alaysha Johnson reported to authorities he had hit her.
The police docs state Johnson told officers she was at a conditioning appointment for an upcoming track and field event when she ran into Fred -- whom she had previously dated for about six months.