Hartley, 27, will be the ninth Kiwi to get behind the wheel of an F1 vehicle and the first since Mike Thackwell in 1984 when he replaces Frenchman Pierre Gasly at the Circuit of the Americas (COTA) track in Austin, the Red Bull-owned team said. 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.
On announcing Sainz's departure, Toro Rosso had stated Gasly would drive in America, but Honda - who supply the Italian team with engines in 2018 - wanted the Frenchman to race the Super Formula finale on the same day for Team Mugen, who they power.
The other vehicle will be driven by Daniil Kvyat, who returns after being dropped in favour of Gasly two races ago.
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"The Austin track is one I really enjoy and one I have raced at recently".
New Zealander Hartley was formerly a Red Bull junior and had previous testing experience with the team in 2009, but his recent career has nearly exclusively involved the World Endurance Championship (WEC), a title he won alongside Mark Webber in 2015.
Along with German Le Mans specialist Timo Bernhard and fellow New Zealander Earl Bamber, Hartley won the jewel in this year's endurance racing crown.
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It seems, though, that a decision has been made, as Toro Rosso team principal Franz Tost is now looking for another driver to fill Gasly's seat.
Hartley will be partnered by Daniil Kvyat in Austin after the Russian was brought back into the Toro Rosso fold following Carlos Sainz's switch to Renault in place of the axed British driver Jolyon Palmer.
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.
"With all the racing experience accumulated over the years, we are convinced that he will do a fantastic job for us", Tost said.
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Hartley, the first New Zealander to earn an F1 drive since Mike Thackwell in 1984, added: "What an awesome feeling".