So THIS is how Selena Gomez celebrated her 26th birthday -- in style, of course, on a yacht, with her friends.

The pop princess appeared to be boating around in Newport Beach with her gaggle of girlfriends and one boy in particular ... savvy friends noticed Selena spending a lot of time with her friend Raquelle Stevens' brother Caleb. Hey, the birthday girl deserves a little romance, especially when the love of her life is engaged to another woman!!!

The group enjoyed Champagne, Italian food, and a spectacular sunset. Meanwhile, the sexy songstress' fans were left wondering ... did Justin Bieber even call to wish Selena a happy birthday?! Perhaps the world will never know ...