The actress/singer didn't hold back one bit in her post, dropping the L-bomb and spilling all the deets about what makes him so damn special to her -- not to mention a bunch of PDA-packed snaps and clips together.

She gushes in the caption, "Happy birthday baby! Your emotional endurance, positive disposition, unbelievable talent (that blows me away), undeniable humor and loving, kind heart absolutely kill me. I love you @itsbennyblanco 🎂🥹."

BB kept it short and simple ... responding in the comments with a flurry of loved-up emojis, which honestly said it all!

As for the pics/clips themselves, Selena's all cuddled up with her music producer boo, stealing kisses left and right ... and giving cute insight into them just being a normal, everyday couple.

Ever since S & B went IG official back in December, they haven't been shy about showing off their love -- and, besides social media, they've also been hitting up award shows and sports events as a duo.

They were also spotted at Disney's California Adventure in Anaheim, CA in Jan ... where we caught the first glimpse of the pair's height difference -- BB's a bit shorter than his GF, but clearly little things like that don't matter to Selena.