Added the insider: "They are both constantly making each other laugh".
Although the brunette beauty seems really loved up with Pete, 24, people are said to be concerned about the authenticity of their romance as she jumped into a relationship with him just days after announcing her split from Mac Miller.
Pete and Ariana have been posting pictures of each other on Instagram since late May.
"They are looking forward to a very long engagement together", a source told Us Weekly. "It's nothing they've been hiding", says a source close to the couple". And now they're engaged. "They are a flawless fit", the source said.
Davidson also got a tattoo of a bunny mask, inspired by the artwork for Grande's 2016 album Dangerous Woman, behind one ear.
Davidson, meanwhile, recently broke up with Cazzie David, the daughter of "Seinfeld" co-creator Larry David. She later described that relationship as "toxic" after a Twitter user wrote it was "heartbreaking" that Miller got a DUI shortly after their breakup.
One user suggested a wedding between Ariana and Pete would be more popular than Meghan and Harry - something Ariana seemed to find amusing.
The pair also reportedly told friends of their engagement as they attended a house party hosted by Robert Pattinson last weekend.