The pop star and actress joined forces as they appeared on Extreme Makeover Home Edition in support of road safety.
The episode was in honour of the Remember Alex Brown Foundation, a non-profit organisation created after 17-year-old Alexandra Brown rolled her utility and died on her way to school while she was texting and driving.
The Brown family - Jeanne and Johnny, along with their 12-year-old daughter Katrina - decided to start the foundation using their home in Lubbock, Texas, as a base.
'I chose to do it that day at the hospital,' Johnny said. 'I decided we have to put the wrecked truck on a trailer and we have to go to schools and tell people about the dangers of texting and driving.'
The Extreme Makeover team rewarded the Browns' charitable efforts by rebuilding their home.
Roberts ditched Hollywood to help after hearing about the cause.
'After hearing Alex's story I just think texting and driving is really a problem,' she said. 'To hear that this happened to someone so young - that really kind of was a little bit of a wake-up call. I have a nine-year-old sister and to think of her losing me, or me losing her - I can't even comprehend that.'
Bieber met the family and promised to donate $1 from every seat sold in his concert that night to the foundation. 'I turn around and there was Justin Bieber,' Katrina said. 'It was so cool.'