One of Young Dolph's suspected killers apparently has some enemies behind bars ... because he was recently attacked by another inmate.
Justin Johnson, who is in custody on a slew of charges including first-degree murder, was punched by a fellow inmate at Tennessee's Shelby County Jail while in the middle of a phone call Friday ... his attorney tells TMZ.
Johnson's lawyer, Juni Ganguli, says JJ was on the phone in the jail's visitation area when an inmate walked up and socked him. We're told there is no word on a motive and Johnson was not severely injured.
A rep for the Shelby County Sheriff's Office tells TMZ ... there was an incident Friday involving Johnson and another inmate, and it's under investigation. We're told there were no reported injuries.
Remember ... Johnson was named as a suspect in Young Dolph's murder earlier this year, and in January he was captured and arrested by U.S. Marshalls. He claims he is innocent.
Johnson's a Memphis-area rapper who uses the stage name Straight Drop ... and he appears to have shot a music video back in the day at the same Tennessee home where police found what they believe is the getaway car used in Dolph's murder.