According to our sources at Rob and Jeanine McCool’s wedding … Bieber and Gomez were strolling on the beach when they heard Justin’s hit “One Less Lonely Girl” playing at the reception — inside the historic and shi shi Adamson House.
We’re told Justin snuck up on some guests — who predictably OMG’d and LOL’d — and then hopped on the mic, saying … “We just crashed it. We heard a party so we decided to just come. So let’s party.”
Beibs and Selena only stayed for about 10 minutes to pose for pics — including some with the happy couple — and then bolted.