In a series of videos posted to Instagram, the "In My Blood" artist and the "Havana" songstress can be seen walking hand-in-hand around a hotel in Tampa, where they smiled for photos and spoke with fans who came up to them to express their appreciation.
Mendes -- who was in Tampa for a performance on Saturday at the Amalie Arena -- spent time with Cabello at the Oxford Exchange, a popular coffee house and bookstore, where they were spotted by many fans.
An eyewitness also told ET that they spotted the two at the Golden Gate Bridge that same day.
"They were hugging and he was resting his head on hers [while waiting for a car]," said the eyewitness, who also shared that a couple of fans approached Mendes and Cabello to say hi and they were very kind and said hi back.
Romance rumors first began after Cabello's recent breakup from Matthew Hussey, whom she dated for more than a year. The two singers then collaborated on a new single -- and sexy video -- "Señorita," and have since been photographed holding hands and spending time together.
The two artists have not yet officially commented on the ongoing romance rumors.