Teen Mom's Boyfriend Arrested For Assault

According to the police report, obtained by TMZ, it all went down on June 16 -- for some reason, Kieffer decided to get aggressive with one of Jenelle's former friends ... and things quickly escalated, with Kieffer PUNCHING both her and her male companion.

According to the report, when police arrived ... the male was "bleeding heavily."

We're told Kieffer -- who's NOT Jenelle's baby daddy -- turned himself in a week later and was arrested for simple assault and assault on a female. He's due in court on August 31.