The hot couple is soaking up the sun from a private perch along a rocky coast in Cala Llenya ... making out above the Mediterranean waves.

Dua's teasing Callum with a skimpy, multicolored bikini ... and he's showing off his toned muscles, wearing nothing but shorts and a bucket hat.

Callum couldn't keep his hands off Dua ... at one point, he crawled on top of her and they started smooching. Hey, ya can't blame him ... she's smoking hot!!!

Dua's on her annual birthday trip to Ibiza, where she's celebrating with a big group of friends ... and this year she brought her boo with her ... looks like she and Callum snuck off here for some alone time.

We've seen Dua and Callum getting handsy all over Hollywood since they first linked up in January ... and now they're taking their relationship overseas.