It's all spelled out in the police report obtained by TMZ Sports ... in which the Delray Beach Police Dept. describe an alleged battery on May 17 at the Buddha Sky Bar.
The alleged victim is 30-year-old Jeff Soulouque -- who says he was hanging at the party when Dennis "unprovokedly turned around and smacked him," right in the face.
Soulouque claims Rodman immediately began apologizing for the incident and invited him out to dinner the following night in an attempted peace offering.
But, Soulouque tells TMZ Sports he was really hurt -- and went to the hospital where he was diagnosed with a left corneal abrasion. He went to cops the day after the incident and filed a report.
Cops launched an investigation and called up a witness named David Lee Roth -- who, shockingly, is NOT the Van Halen singer. Still, cops say DLR corroborated Soulouque's story.
"Roth stated he was about twenty feet away and he saw Rodman turn around and open hand smack Soulouque two times," the report states.
Cops are now on the hunt for video of the incident and have listed Rodman as an official suspect in an alleged battery.
As for Rodman, his attorney, Lorne Berkeley, denies his client slapped anyone, telling TMZ Sports ... “I’ve spoken with people who were with Dennis the entire night of the alleged incident at Buddha Sky Bar. No one saw any such incident occur and Dennis was sober the entire night.”
“Currently the matter is in its preliminary stages but we fully expect after a thorough investigation that no such incident will be substantiated. Dennis denies any such incident occurred.”
It's just the latest in a string of bizarre incidents involving the NBA legend -- who is also being investigated in California for his role in an alleged clothing heist at a Newport Beach yoga studio.