Justin Bieber -- Fan Says 'I Got Coldcocked' for Taking a Pic

Justin Bieber may have turned over a new leaf, but not everyone got the memo ... because TMZ has learned a guy filed a police report ... claiming he was decked by security just for snapping a pic.

Steven Prince tells TMZ he was at FLUXX, a San Diego nightclub, Sunday to watch T.I. perform. He spotted Bieber in a VIP area and went over to take a photo.

Prince claims a security guard screamed at him and began shoving him, then punched him twice in the face -- on the cheek and jaw.

The guy claims the club tried to make nice by offering him a free drink, but then kicked him out.

Prince filed a police report and -- surprise -- he's lawyered up and plans to sue.