Ah, young love!

Justin Bieber and Selena Gomez weren't afraid to flaunt their affection before a crowd of thousands -- and an audience of millions -- at Sunday's Billboard Music Awards.

When Bieber, 17, won a a trophy for Best New Artist, his seat mate Gomez, 18, (clad in a sexy Dolce and Gabbana gown) gave him a big wet smooch on the lips and a warm hug.

"I'm 17 and I want to thank God because he has blessed me so much," Bieber said at the podium as Gomez and his mother looked on proudly.

It's the most high-profile kiss yet for the power-couple, who came clean with their romance earlier this year -- holding hands at the Vanity Fair Oscars party in West Hollywood.