Jagged Edge got bounced from Jermaine Dupri's birthday bash at a swanky Atlanta club ... because the guys refused to ditch their ball caps at the door.
Gold Room host Kenny Burns tells TMZ ... the R&B group showed up under-dressed -- hoodies and caps -- for the exclusive event. Owner Alex Gidewon tried to compromise -- since it was Jermaine's party ... agreeing to let the guys in IF they lost the hats.
We're told when Jagged Edge refused, they got the boot -- which pissed off Jermaine BIG time ... and the guys started cussing out security. Eventually they all got the heave ho.
JD tells us he and his crew, including Jagged Edge, were in the club for 35 minutes -- without any problems -- before management ever complained. He feels the club was very disrespectful.
The club seems willing to forgive and forget with Jermaine ... but we're told Jagged Edge has been banned from the Gold Room.