Pattie Mallette is fully supportive of her 18-year-old son's relationship with the 20-year-old singer-and-actress, and thinks the couple are "great together".
She said: "How can you not like her? She's so sweet and she's good for him and they're great together. They're adorable."
The 37-year-old single mother made a vow aged 21 that she wouldn't have sex until she got married, and she promised Justin that she wouldn't get a boyfriend until he was 18.
Pattie was just 18 when she fell pregnant and brought Justin up with the help of her parents after splitting from Justin's father Jeremy Bieber.
She explained: "I take that vow really seriously. I remember when I made it, I wrote it down on a piece of paper, I had someone witness it and it means a lot. I made it this far, I'm not stopping now.
"When I moved to Atlanta with Justin, because of the all the chaos and how crazy it was, I told him I would focus on him and wait until he's 18 before I started dating. Now he's just turned 18."
Pattie admits she had a "toxic relationship" with Jeremy - who has gone on to marry and have two children - when she was younger but she now considers him to be a "friend" and insists he is a "good dad" to Justin.
Speaking on 'The Ellen DeGeneres Show', she explained: "Me and his dad had a toxic relationship growing up, we were teenagers, it was on again, off again. But as we got older we both grew and his dad has been around since he was a baby.
"Some people think that he just came along when he became famous but that's not true. He's a good dad, and I consider him a friend today.
"We went to Disney a couple of weeks ago all together and we had a good time. Justin loves Disney."