"Was gonna wait a while to say anything but word travels fast, listen plain and simple Hailey I am soooo in love with everything about you!" the singer wrote beneath a black-and-white photo of Baldwin kissing his cheek.
"My heart is COMPLETELY and FULLY YOURS and I will ALWAYS put you first! You are the love of my life Hailey Baldwin and I wouldn't want to spend it with anybody else," he continued.
Bieber also seemed to confirm that he popped the question on Saturday, writing that the couple got engaged on the seventh day of the seventh month.
Seven "is the number of spiritual perfection, it's true GOOGLE IT!" he added.
Baldwin quickly followed up Bieber's Instagram post with a tweet of her own.
"Not sure what I did in life to deserve such happiness but I am so utterly grateful to God for giving me such an incredible person to share my life with," the 21-year-old model wrote.
The duo, who briefly dated in 2015 and 2016, got back together about one month ago. Multiple outlets reported Sunday that Bieber proposed at a restaurant in the Bahamas.