Justin Bieber and Selena Gomez Crash Wedding

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."