The 24-year-old actress stepped out in style, rocking a sunny yellow gown, to show her support for her boyfriend at the star-studded premiere of his new romantic drama, The Sun Is Also a Star.
Speaking with ET's Leanna Aguilera outside the Pacific Theaters at the Grove in West Hollywood on Monday, Mendes opened up about getting the chance to celebrate Melton's big day.
"It means so much!" Mendes shared. "I feel like I've been hearing about this project since forever, so to finally see all of this come to fruition is a beautiful thing."
Speaking with ET at a junket for The Sun Is Also a Star, Melton revealed that he occasionally writes love letters to his girlfriend, which Mendes gleefully and adoringly admitted was true.
"He does! He's a hopeless romantic for sure!" she shared.
The Riverdale co-stars have made no secret of their real-life romance, and have used every opportunity to express their love -- sometimes in a (sort of) permanent way!
Back in February, Melton showed off a "Cami" tattoo -- which is short for his girlfriend's name -- which he got inked on his chest. (However, Mendes later revealed that the tattoo was just a fake stick-on.)
To hear more about the cute couple from Melton himself -- and his penchant for penning heartfelt letters to his ladylove -- check out the video below.