On Friday, the 22-year-old singer took to Instagram to share a throwback Polaroid pic of her and Mendes in honor of his 21st birthday. In the silly snap, Cabello is smiling as Mendes plays with her hand.
"Happy birthday to this magic human, I love you!!!!❤️❤️❤️❤️," she gushed in the caption of her rumored beau.
The sweet post comes one day after an eyewitness told ET that Mendes celebrated the milestone occasion in New York, where he rented out Harriet's Rooftop & Lounge at 1 Hotel Brooklyn Bridge.
The bash got underway around 10 p.m., with the attendees singing the birthday boy "Happy Birthday" as he was presented with a cake, which featured replicas of the essential items found in his dressing room on tour, the eyewitness said.
Mendes appeared to have fun at the party, as he told his guests that he was too drunk to give a speech after they sang to him. According to the eyewitness, Cabello stuck by Mendes' side for much of the party -- which also included beer pong and dancing -- before the two shared a steamy kiss and the party wrapped up around 2 a.m.
"Shawn and Camila originally were just enjoying spending time together and having a summer fling," the source said. "Camila had just got out of a relationship and Shawn was in the middle of a massive tour."
"However, Shawn and Camila have really fallen for one another," the source added, noting that the former Fifth Harmony star has been traveling with Mendes throughout his tour.