The couple, who dated in 2016 before splitting, reportedly got engaged in the Bahamas on Saturday night.
Bieber's father Jeremy addressed his son on Instagram, writing "proud is an understatement! Excited for the next chapter!"
Pattie Mallette, Bieber's mother, also appeared to hint at the engagement on social media.
Love Love Love Love Love Love Love.— Pattie Mallette (@pattiemallette) July 7, 2018
Two eyewitnesses reportedly said they were at a restaurant in the Bahamas when Bieber's security told everyone to put their phones away as something special was about to happen.
Hailey Baldwin is the daughter of actor Stephen Baldwin.