Following her split from Matthew Hussey, Cabello and Mendes raised eyebrows with their steamy "Señorita" music video.
The singers fueled romance rumors with a PDA-filled brunch in West Hollywood in early July.
"It already feels to them like they've been together for a lot longer than they have," a source told E! News in mid-July. "They've known each other and been a part of each other's lives for a while so it's all very natural and moving quickly."
"The timing wasn't right before but it is right right now," the insider continued. "They are going for it without holding back. This step in their relationship is exciting for them both. They haven't wanted to be apart at all. It's 24/7 and all out at this point."
In late July, the loved-up duo had a makeout session in the water while in Miami.
The couple, in their swimwear attire, held hands as they spent time with pals in the Florida city.
Cabello was by her man's side for his 21st birthday party in early August, even sharing a passionate kiss at the bash.
The duo enjoyed a date night in Montreal in mid-August, with a source telling E! News that Cabello and Mendes "couldn't keep their hands off each other."