TMZ reported the singer proposed to his girlfriend Saturday night at a restaurant in a resort in the Bahamas.
Bieber, 24, and Baldwin, 21, have been a couple for about a month after previously dating and breaking up over the past couple of years.
"It's kind of a surprise, but kind of not. Justin has been extremely happy these past few weeks," a source told People.com. "He has known Hailey for a long time. This might seem sudden, but they know each other very well."
The celebrities have not publicly announced their engagement, but UsMagazine.com said it also confirmed Bieber proposed marriage to Baldwin and she accepted.
Bieber's parents posted social-media messages that are being interpreted as confirmation, as well.
"@justinbieber Proud is an understatement! Excited for the next chapter!," Jeremy Bieber captioned a photo of his son looking out at the ocean at sunset.
"Love Love Love Love Love Love Love," his mom Patti Mallette tweeted.