Mack Maine -- Surrenders to Jail in Tour Bus Assault Case

Rapper Mack Maine is facing the music ... he turned himself in to the Oklahoma County Sheriff's Office early Friday morning to face a felony charge for allegedly assaulting two women on his tour bus ... TMZ has learned.

Mack was booked in Oklahoma City for aggravated assault and battery with great bodily injury -- in connection with an alleged incident that went down on August 22.

As TMZ first reported ... one of the women claims Mack stuck his hand down her shirt and grabbed her breast -- while the other woman alleges he punched her in the face ... when both women were attempting to get off the bus.

Mack is currently behind bars ... and his bond has been set at $9,000.