Justina Valentine is 39 and looking great ... and if you need receipts, this gallery’s got 'em!

The birthday babe knows exactly how to crank up the heat -- and it’s not just the signature red hair, 'cause those barely-there bikinis are doing all the talking and showing off every curve.

Justina’s body confidence is on full blast, keeping it on-brand in fiery red ... no matter what corner of the globe she’s posing in.

So go ahead -- scroll, stare, and celebrate. Justina’s blowing out candles and serving looks. Happy Birthday!