The 22-year-old joins Robert Kubica as a reserve at Alfa Romeo, meaning he will step into the breach when the Pole is unavailable due to other commitments.
It has already been confirmed that Ilott will be behind the wheel this weekend in the opening practice session at the Portuguese Grand Prix, the first of “a number” of such drives in 2021.
That will, however, be his third outing for the squad, following tests in 2019 and the 2020 post-season affair at Abu Dhabi.
“I am really happy to be joining the team for this season and I'd like to thank Alfa Romeo Racing Orlen and the Ferrari Driver Academy for the trust they have put in me,” said the Briton.
“The two sessions I had with the team in the last two years have been extremely useful in getting me accustomed to the way a Formula 1 team works and I am confident I can hit the ground running in my new role as one of the reserve drivers at Alfa Romeo Racing Orlen.
“I can't wait to be in the car and help the team continue its progress.”
Alfa Romeo team principal Frederic Vasseur noted the positive impression which Ilott had made in his previous interactions with the squad.
“I am delighted to welcome Callum to our team: his journey through the junior series has been an impressive one and he is without doubt one of the most talented young drivers coming through the ranks,” said Vasseur.
“He has worked with us before, each time leaving a lasting impression thanks to his work ethic and good feedback, and I have no doubt he will be a very positive addition to Robert Kubica, who will be unavailable on various occasions due to his other racing programmes.
“Callum will have a busy programme and we can't wait to see him on track again this weekend.”
Alfa Romeo is currently eighth in the constructors' championship, without a point to its name, after the first two rounds of the current campaign.
Kimi Raikkonen had initially finished ninth in the most recent race, at Imola, but was subsequently issued a 30-second penalty having spun and failed to retake his position in time for the post-red flag restart.
It has not yet been announced whether Ilott will replace Raikkonen or Antonio Giovinazzi when Practice 1 at Portimao commences this Friday at 20:30 AEST.