The 25-year-old shared a photograph of the couple kissing on a boat, with the sun setting over the ocean in the background.
"365 days. i love you," Mendes captioned the shot.
Melton also marked his romance with his girlfriend, and posted a similar shot of them cuddling on a boat. "I love you," he simply wrote alongside the photo.
Mendes previously opened up about her relationship with "The Sun Is Also a Star" actor, and told Entertainment Tonight that Melton frequently writes her love letters.
"He's a hopeless romantic, for sure," the "Riverdale" star gushed.
The couple confirmed their romance in October last year (18) with Mendes sharing a picture of the couple embracing. "Mine," she captioned the snap.