The move, which comes amid a difficult season for Kvyat during which he has scored just four points to Sainz’s 48, will allow Toro Rosso and parent company Red Bull the chance to make a more informed decision on their 2018 driver line-up, according to team principal Franz Tost.
"Scuderia Toro Rosso was established by Red Bull to bring youngsters from its Junior Programme into Formula 1 and that's what we are doing by giving Pierre this chance,” said Tost.
“He is the next in line at Red Bull for this opportunity and he has shown that he deserves it, having taken the 2016 GP2 title and this year being very competitive in the Super Formula series in Japan. He really has a valid chance of winning the title, as he is only half a point behind the leader.
“The driver switch gives us an opportunity to make a more informed decision regarding our 2018 driver choices. For a variety of reasons, some of them due to technical problems, but others being mistakes of his own making, Daniil Kvyat has not really shown his true potential so far this year, which is why we are standing him down for the next races. This will give us the opportunity to evaluate Pierre on track during a proper race weekend."
Gasly, 21, has acted as Red Bull’s reserve driver in 2017, driving the RB13 at the post-race tests in Bahrain and Hungary. He currently lies second in the Japanese Super Formula standings with two races at Suzuka remaining, but as well as going for that crown he will also now have the chance to prove he belongs in F1.
"I would like to thank everyone who has helped me get this chance in my motor racing career and specifically, Red Bull, Dr. Helmut Marko and Scuderia Toro Rosso,” said the Frenchman.
“This is a great opportunity for me. I feel as ready as I can be, having had to be prepared for anything in my role as Red Bull Racing's third driver this year. I will do my best to perform well with Scuderia Toro Rosso during these coming races."
In a statement, Toro Rosso added that the driver switch should not be considered a permanent parting of the ways with Kvyat, saying "This is not a case of goodbye for our Daniil, as he still remains part of the Red Bull Family."
However, Toro Rosso's ambiguity over how many races Gasly will remain in the car means it is not currently known if and when Kvyat will return to the cockpit, though the fact that the Super Formula season finale takes place on the same weekend as the United States Grand Prix suggests the Russian may return for Austin.