di Giannantonio lost his Gresini seat to Marc Marquez and, although denied a direct swap with the Spaniard, the Qatar winner will now fill the void left by Luca Marini's own Repsol Honda move.
The #49's Gresini seat was under threat even before the shock Marquez rumours, but everything began falling into place performance-wise in the last seven rounds.
It started with a career-best equalling eighth in Japan, followed by fourth in India, a debut podium in Phillip Island, runner-up in the Qatar Sprint and fairytale victory in the main grand prix.
di Giannantonio then came within a fraction of back-to-back Sunday wins at the Valencia finale, although he was demoted from second to fourth by a post-race tyre pressure penalty.
Diggia’s surge in performance propelled him past VR46's initial replacement targets, Moto2 riders Fermin Aldeguer and Tony Arbolino, to seal the GP23 ride alongside Marco Bezzecchi.
"I'm happy to join such a strong Team as the Mooney VR46 Racing Team, I can learn a lot and I can't wait to start working together on this project, we can do really good things," di Giannantonio said.
"In the last two seasons in MotoGP I have grown a lot and in this Team I will certainly be able to mature further and achieve great results. I'm excited, I can't wait to meet all my new staff and crew and start working on the track. Thanks to everyone who made this possible."
di Giannantonio is the first rider to join Valentino Rossi's MotoGP team who is not a member of the VR46 Academy, or indeed a rookie (as both Marini and Bezzecchi had been).
"I am really happy to announce that Fabio will join our team," Salucci said. "It was an unexpected arrival, at the last minute but I am satisfied that we have chosen him. He had a great end of the season, he didn't lose himself when things didn't go well and I'm sure he'll be able to feel at home here.
"On a motorsport level, he has proven to be among the fastest young Italians, he will continue to work with Ducati and the objective is to help him be not only competitive, but also consistent in his results.
"It is an ambitious project, it marks a turning point in the history of our Team, but I am sure that we will all give our best to go in the same direction."
di Giannantonio, who finished twelfth in this year’s world championship, will make his VR46 debut at Tuesday’s post-race test.