Justin and Hailey were leaving dinner at Cecconi's Tuesday night in Brooklyn walking arm-in-arm ... and getting bombarded by fans and photogs. Hailey made for a quick escape into their waiting car, but Biebs was super friendly -- talking to fans, photogs -- and even taking a trip down memory lane with some locals.

The two, who dated briefly in 2015, were first spotted back together in Miami over the weekend. Looks like they can't quit each other now that their romance has moved to the Big Apple.

It's cool though ... we like this Justin a lot more.