She also took a moment to thank Jolly Old Saint Nick for bringing the 22-year-old singer-songwriter into her life.
"Thank u Santa for my tall sweet funny boy," Cabello wrote.
The couple is currently spending the holiday season at Mendes' home in Canada. On Christmas Eve, he took to his Instagram Stories to share an adorable photo of the pair cozying up together on the couch with their dogs.
In early December, Cabello and Mendes gifted fans with a duet of the holiday classic "The Christmas Song." The accompanying video features vintage-style footage of the couple showing off sweet PDA, looking at Christmas lights and playing with their dog Tarzan.
"The Christmas Song" is featured on the holiday deluxe edition of Mendes' new album, Wonder.
Check out Cabello and Mendes' steamy Christmas kiss on Instagram here.