The team has released a statement confirming his appointment for that race only.
Hartley will drive at the US Grand Prix in what will be a long-awaited debut for the New Zealander.
One might wonder why Toro Rosso didn't simply hold on to Sainz for just one more round, which would have allowed Gasly to compete in the Super Formula finale and eliminated the need to find a fourth available driver.
According to multiple reports, the 27-year-old will race for the Toro Rosso Formula One team in the United States next weekend with a possibility of racing with the team on a permanent basis in 2018.
Who that replacement is, though, has yet to be revealed.
He has raced in the Le Mans 24 Hours each year since, while also competing in the FIA World Endurance Championship since 2014.
"This opportunity came as somewhat of a surprise, but I never did give up on my ambition and childhood dream to reach F1".
Hartley is contracted to Porsche in the World Endurance Championship (WEC).
Because Kvyat was put on hiatus by Toro Rosso because of his poor on-track performances, you'd imagine he doesn't have much leverage: but teams are contractually obliged to run two drivers and no more than four per season.
He went on to fashion a terrific record in GT racing, including his win with the Porsche factory team at Le Mans this year.
Honda, who will provide the engine power for the Faenza based squad in 2018, had also stated they would not stop Gasly from racing in the USA, but would be happy for him to return to Japan if a replacement Formula 1 driver could be sought.
If Hartley does race, he will become New Zealand's first Formula One driver since Mike Thackwell in 1984. "With all the racing experience accumulated over the years, we are convinced that he will do a fantastic job for us".
There hasn't been a Kiwi driver in the world's premier motor racing contest since Mike Thackwell in 1984 but that is set to change, with Hartley, the Le Mans 24 Hour victor, tipped for the vacant Toro Rosso seat at next week's US Grand Prix.
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