Brit and Sam Asghari boarded a private jet for parts unknown ... and the celebration got underway even before they fastened their seatbelts.

The plane was adorned with bouquets of roses, but that was just the beginning. There was a pretty incredible birthday cake ... white meringue if you want to know. Her reaction was, "Oh my God!' as he sang happy birthday. There were birthday cards and when they landed there was a big smooch for their big exit.

It was all followed by fireworks ... and if you look closely you see 2 lounge chairs, so it seems they're at some sort of beach resort. Britney's go-to is Maui, so that would be an excellent guess.

Sam posted this ... "I call you Lioness because I admire your relentless strength, I'm inspired by your beautiful heart, I celebrate your smile that lights up my world ❤️ Everyday is your birthday my queen 👑 Happy 1st birthday to my wife."

It's interesting he referred to her as his "wife" ... don't read too much into it. He's said this before. There are things that have to happen before they tie the knot ... like a prenup.