Daniel Riccardo is going to be back racing in Formula 1
The 34-year-old Australian, who was released by McLaren last year, will race at the Hungarian Grand Prix on 23 July.
He replaces Nyck de Vries, who has been dropped after only 10 races.
Ricciardo, who has won eight grands prix, rejoined Red Bull as a reserve driver at the beginning of this season.
Dutch rookie De Vries, 28, has lost his drive after failing to pick up a single point, as well as having several crashes and struggling for form in comparison with younger team-mate Yuki Tsunoda.
Ricciardo took part in a tyre test for Red Bull at Silverstone on Tuesday, two days after De Vries finished last in the British Grand Prix.
"I'm stoked to be back on track with the Red Bull family," said Ricciardo.
Alpha Tauri team principal Franz Tost said: "I'm very pleased to welcome Daniel back into the team.
"There's no doubt about his driving skill, and he already knows many of us, so his integration will be easy and straightforward.
"The team will also profit a lot from his experience, as he is an eight-time Formula 1 grand prix winner.
"I would like to thank Nyck for his valuable contribution during his time with Scuderia Alpha Tauri, and I wish him all the best for the future."
Ricciardo's career in motorsport's most illustrious category was looked over when McLaren terminated his contract a year early due to what they considered unsatisfactory performances, choosing to replace him with compatriot Oscar Piastri.
But he rejoined Red Bull, for whom he won seven grands prix between 2014 and 2018, as their third driver at the end of the season and is now back in a full-time race seat after less than half a season out of the action.
-- Courtesy of BBC Sport