Sainz drove for Scuderia Toro Rosso in the season where he partnered Max Verstappen, following Daniil Kvyat's promotion to Red Bull. Sainz selected 55 as his race number. He qualified inside the top ten for his début, at the 2015 Australian Grand Prix, and finished the race in ninth position. In the third free practice session of the 2015 Russian Grand Prix, Sainz lost control of his car going into turn 13 of the Sochi Autodrom, hit a wall and went on to crash into the Tecpro barriers. After spending a night in hospital, he was allowed to race the next day but did not finish the race. In the United States Grand Prix he went to finish 7th which was his last points finish of the year. He finished 15th in his first season with Toro Rosso.