Kylie Jenner just boarded a private plane to her birthday yacht party with a very interesting piece of luggage ... a garment that certainly looks like a WEDDING DRESS ... but TMZ knows otherwise.
Check out this pic from the tarmac Tuesday afternoon at the Van Nuys Airport ... there's a flowing, feathery, white dress protruding out the bottom of a garment bag as it's loaded into the plane. In the worker's other hand -- what appears to be a suit ... possibly a tux.
Noticeably absent on the flight -- Caitlyn Jenner and all of Kylie's sisters. Of course, Kourtney's been vacationing in Europe, and it seems she'll join the party at some point.
One source says it's definitely NOT a wedding because Kim and Khloe are not going to join on the trip.
You could argue ... Travis set the stage when he gifted Kylie a garden of rose petals Monday, covering the floor of her mansion. Travis wrote, "We're just getting started." She also got a $21,000 custom floral arrangement from La Fleur Bouquets inspired by the Kaws x Dior collab from her best friend Yris Palmer.
FYI ... Kylie and Travis have been together for a little over 2 years. Marriage is certainly in the cards ... just not on this trip, we're told.