The `Baby` hitmaker explained his mother, Pattie Mallette - who raised the 17-year-old singer singlehandedly - wanted to be "straight up and honest" with him, and was concerned he would think she was a "liar" when he later found out Father Christmas wasn`t real.
He said: "My mom always told me there wasn`t a Santa. This was her logic: she thought if I grew up knowing about Santa then finding out he wasn`t real, that it would be like she was lying to me. And then when she told me about God, I maybe wouldn`t believe her.
"She wanted to be straight-up and honest with me all the time. I didn`t tell my friends or ruin it for anyone - I was a good kid!"
Justin - who is dating singer-and-actress Selena Gomez - admitted his only Christmas wish this year is for his family to be happy and healthy.
He added in an interview with AOL Music: "To be honest, I really don`t have any Christmas wishes this year. I just wish to be happy and that everyone be healthy in my family."