The popstars, who dated on-and-off between 2011 and 2014, were spotted at Gomez's home in California over the weekend.
In photos obtained by TMZ, Gomez, 25, is seen walking into her house about 10 minutes after Bieber, 23.
They were reportedly joined by other mutual friends.
Gomez's boyfriend The Weekend, who is currently on a world tour, was aware of the reunion.
Bieber and Gomez have both opened up about their past relationship, with the 'Sorry' singer saying he will always love his ex.
“It was really tough. I don’t know if I’m over it yet,” he told Access Hollywood in 2015. “[She] is someone I love dearly. I’m never going to stop loving her. I’m never going to stop checking in on her."