The "Attention" hitmaker first teased fans about the "really special" collaboration back in May, when he posted a photo of himself with the "Rocket Man" icon on Instagram.
Puth reveals the track is now finished and will be released "soon," and getting to hit the studio with his music idol was really a dream come true.
Marvelling at having Sir Elton's contact details, he told U.S. breakfast show "Today", "He just emailed me, which is pretty cool. It's pretty cool seeing 'Elton John' pop up on your phone."
Puth reveals the two musicians are neighbors in Los Angeles, and although they had discussed the idea of collaborating, the 28-year-old never thought it would actually come to fruition.
"We met at a restaurant in L.A. - this was back in December - and he said, 'Yeah, let's write,' and I didn't expect him to hit me up the week after, and be like, 'We're neighbors, we should just use your studio...' "
The artists quickly discovered they had completely different methods of songwriting, "He said to me something really interesting, he said, 'You know, Charlie, I write music when lyrics are given to me, and you write lyrics after,' which is true, so we write very differently, but he said, 'Let's take a crack at it anyway.' "
And their efforts paid off. "(Elton) sat down and played the entire three minute and three second song - that is done and people will hear soon. He played it in one take, the piano part," he shared, "and then I wrote the lyrics. We did it reversed..."