Jennifer Lopez's Butt -- Just a Reminder: My Backside's Outta Sight!

Jennifer Lopez did her first ever concert in her hometown of the Bronx last night, and she did not hold back the goods ... giving the crowd a view of her prized backside.

J.Lo's free show at Orchard Beach -- in advance of her new album coming out later this month -- featured actual singing and multiple costume changes ... most of which showed off her rear end, thankfully.

The butt show was clearly a response to the latest challenger to the ass throne ... Bradley Cooper.