Although he has yet to actually follow her on Instagram, the 24-year-old singer spent some time scrolling through her feed on Wednesday night.
It appears Bieber couldn't help himself from leaving a comment on the 21-year-old model's latest glamour shot, in which she's rocking a sultry smoky eye with her blonde locks styled into big, bombshell waves.
"Dats mine," Bieber gushed of a pic she posted on July 4.
Earlier this week, Baldwin had a little fun of her own on Instagram, slightly poking fun at the scruff her future husband was sporting in the black-and-white pics he posted to confirm their engagement.
When Bieber's close friend, Ryan Good, commented, "Grow the mustache back for the wedding," Baldwin replied, "Don't u dare give him that idea you lunatic."
As ET previously reported, Bieber proposed to Baldwin over the weekend while the two were vacationing in the Bahamas. Before the pop star got down on one knee, he made sure to get approval from Baldwin's father, actor Stephen Baldwin, who gave him his blessing to marry his daughter during a meeting in New York City, a source told ET.