Hartley, this year's Le Mans 24 Hours victor and a world endurance champion, will add his name to a roll call of Kiwi Formula One drivers that now ends with Mike Thackwell in 1984.
"What an awesome feeling, I never did give up on my ambition and childhood dream to reach F1", said Hartley, who won the 24 Hours of Le Mans in June.
Toro Rosso team principal Franz Tost pointed out that Hartley had longstanding Red Bull roots, and also hinted that the team - which tends to try to provide young talents with a springboard towards the Red Bull team - was eyeing a more experience pair of hands for Austin. If Toro Rosso and Hartley can't successfully come to an arrangement, the team will look to Sebastian Buemi, who previously raced for the Red Bull Junior Team as well.
The rookie, who is set to return for the last three rounds of the season after Austin, has said he would rather stay with Toro Rosso and the team indicated that would be the case when he arrived.
Hartley is leading the world endurance series again this year and could become champion at this weekend's race at the Fuji Speedway in Japan.
He can win a second World Endurance Championship with the factory Porsche team if results go his way this weekend in Japan. "I'm trying not to put too many expectations on my F1 debut, but I feel ready for it".
"What an unbelievable feeling!", Hartley said.
You'll note that the other driver can't be Kvyat: he has already returned in the other vehicle, having replaced Carlos Sainz Jr, who went off to Renault as part of a new engine deal between Honda and the two teams.
And while it might seem likely that Kvyat will stay on, he has now been demoted twice by Red Bull - once from the senior team back to Toro Rosso in May 2016 and then dropped altogether after this year's Singapore event. We are really happy to have him back in our team.
He becomes the first New Zealander since Mike Thackwell in 1984 to compete in motorsport's premier category and just the ninth overall. "With all the racing experience accumulated over the years, we are convinced that he will do a fantastic job for us", he said.