The Weeknd, Bella Hadid -- In Case You Didn't Notice ... We're Together

The Weeknd and Bella Hadid made it photographically official Tuesday night ... leaving a club hand in hand.

They've been dating on the down-low for 4 months but last night clearly wanted it known ... they're together as they locked hands leaving Up and Down Nightclub in NYC.

A little background ... she's 18, he's 25.

Good looking couple for sure.