Carlos, who will be ceded by Red Bull, with whom he has contract until end of 2018, replaces British Jolyon Palmer, who has finally accepted economic proposal of Renault to leave early, and share team with German Nico Hulkenberg.
"The Japanese Grand Prix will be my last race for Renault", Palmer said.
Additionally, competing in these last four races with Renault gives me a good opportunity to get to know the team and the auto earlier than expected.
"I would like to thank Carlos for all his efforts over nearly three Formula 1 seasons", said Toro Rosso team boss Franz Tost.
"When I'm heading back on the plane maybe it will sink in that I'm not driving for Renault again and maybe not in Formula 1 again".
Toro Rosso's Carlos Sainz Jr. walks after crashing during practice.
Daniil Kvyat will return to Toro Rosso beside Pierre Gasly for the remaining four races of the season.
"First of all I would like to thank Toro Rosso and Red Bull for allowing me the possibility of finishing this season with Renault Sport Racing", Sainz said. "I've been with the team now for three years so it's a shame not to be going to Austin with them".
"On my side, I will definitely do my very best in tomorrow's race - this would be the best possible send-off!" "I think this chance gives me the opportunity to get to know everyone inside the team, start working with my engineers, try to adapt the auto to myself and myself to the vehicle".
Toro Rosso boss Franz Tost gave Sainz a glowing reference worthy for his resume, even though he already landed the Renault seat.
"I would like to thank Carlos for all his efforts over nearly three Formula 1 seasons", Tost said.
"He has always shown great human qualities and we wish him the best for the rest of his career".
Meanwhile, Sainz's early Toro Rosso exit means Gasly, who made his F1 debut in Malaysia last week, will miss out on the chance to win the Japanese Super Formula series later this month.
"The results on the timesheet look worse than what it really is, because in Q2 we decided not to run any new tyres - we only went out on the ones used in Q1", said Sainz.