The 23-year-old has been allowed to move to the French manufacturer as part of the complex negotiations which will see Red Bull switch to Honda power units from 2018, while Renault will supply McLaren with their engines.
The Spaniard has made no secret of his desire to move up the grid having impressed for Red Bull's junior team Toro Rosso this season, scoring 36 of their 40 points.
However, he will remain contracted to Red Bull, who have only allowed him to move on loan to Renault for the duration of the 2018 campaign and appear keen to have him back once he has gained more experience elsewhere.
Despite not being allowed to join the Enstone-based outfit permanently, Sainz is excited about the direction his Formula 1 career is headed.
"This is the start of a very exciting new chapter in my career. I would like to say thank you to Red Bull for all their confidence and support and for allowing me to take this opportunity," he said.
Sainz's arrival means Jolyon Palmer will need to look for a new home for next season, the Brit having struggled to impress for Renault since their return to F1, failing to score a point this season while team-mate Nico Hulkenberg has 36 to his name.