The couple were having a romantic dinner date at a restaurant on Malibu beach when they heard Justin`s music playing at some nearby nuptials.
So they ventured over, leaving the bride and groom and their guests stunned.
A source said: "Unbeknownst to all of the guests, JustinBieber and his girlfriend SelenaGomez were just done eating at a restaurant up the road from the Adamson House estate and had decided to take a stroll down the beach. I guess he heard his song play and decided to crash the wedding party to check the scene out. He was cool enough to make a cameo, which surprised all of the wedding guests."
The `Baby` singer also congratulated the newlyweds, Rob and Jeanine McCool, on tying the knot and posed for pictures with the couple and guests.
The source added to PerezHilton.com: "He even took the microphone, congratulated the newlywed couple, and said a few nice words, while posing for a series of pictures with his fans. That was one of the coolest and nicest things a person of his celebrity can do, and it certainly made a memorable night for the newlywed couple Mrs. and Mrs. Rob and Jeanine McCool."