When in Bali, do as Alessandra Ambrosio does ... immerse yourself in the lush tropical landscape, sip on a fresh coconut, and strut your stuff in a thong bikini like it's your day job!

No questions asked, the 45-year-old model made sure that bikini got its moment in the sun ... and she made sure to document it for social media.

And obviously, if you've got your bae with you, pack on the PDA for a smoochin' selfie ... Ambrosio announced her engagement to Buck Palmer back in August, and they've since continued their worldwide love tour!

Check out the gallery ... Alessandra's not afraid to turn every destination into her own personal bikini runway!