Jennifer Lopez Wants T Remind You That Her Butt Is Out Of 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. 

One word, Cooper -- fishnets