The 19-year-old singer shared the picture on Instagram amid rumors he's dating the 21-year-old model.
The snapshot shows Mendes and Baldwin, the daughter of actor Stephen Baldwin, striking a pose as they gaze at the camera. Mendes captioned the photo with two camera emojis.
Mendes' post had received over 3 million likes as of Wednesday morning, including from Baldwin and celebrity friends Paris Jackson, Calvin Harris and Rita Ora. Singer Julia Michaels also commented with a red heart emoji.
Mendes and Baldwin have been linked since the fall, although the model told Us Weekly at the iHeartRadio Music Awards in March that she's "not" dating anyone. She told People prior to the awards show that she laughs off "most" dating rumors.
"It's quite funny to me, to be honest," the star, who previously dated Justin Bieber, said. "I try to keep that side of my life as private as possible. I don't feel like I have to explain myself to people because it's my life."
E! News previously reported Mendes and Baldwin are "definitely" dating.
"It started out casual as they would run into each other at parties and hangout, but most recently it's become more serious," a source said. "They are definitely dating and spend a lot of time together."
"Their relationship has grown and they don't care if anyone sees them now," the insider added.