Selena Gomez Says She's 'Excited' About Wedding on 'Tonight Show'

Selena Gomez can hardly contain her joy about marrying Benny Blanco ... grinning ear-to-ear while talking about their upcoming wedding on "The Tonight Show."

On Tuesday, she appeared alongside her "Only Murders in the Building" costars Steve Martin and Martin Short, when host Jimmy Fallon brought up that her wedding is right around the corner.

The audience erupts ... and Selena flashes a big smile, chuckling as the crowd screams with excitement.

Fallon follows up by asking if she's having fun planning the wedding -- and Selena lights up, saying, "It's wonderful. I'm very lucky. It's going well. I'm so excited."

Steve Martin cracks a joke ... saying he's sure his and Short's wedding invites will be arriving any day now.

Selena confirms both of them are invited, and quips that Short will be the ring bearer. Short then pokes fun, "We are all so excited because we love this lady, and we love her fiancé Bad Bunny."

TMZ broke the story ... Selena and Benny are set to say "I do" at the end of September, with a swanky wedding going down in the Montecito/Santa Barbara area of SoCal. The venue's being kept on the down low -- even the guests don't know where it is yet.