In 2013, Sainz signed with Arden to compete in the GP3 series. During the first qualifying session of the year, he qualified in 5th place, 5 tenths off of pole sitter Kevin Korjus. However, he, as well as Alex Fontana and Patrick Kujala, were penalised 10 places for ignoring yellow flags during free practice. During the beginning of race one, Sainz got up to 13th place by passing Alex Fontana and Jimmy Eriksson off the start. By the start of lap 3, he was 13 seconds behind the leader. Sainz made it up to 8th place with his teammate Daniil Kvyat until both cars lacked grip. Because they were both pushing to get into the top ten for points, neither driver decided to manage their Pirelli tyres and so by the end of the race, Kvyat was 20th and Sainz was 15th. He finished 51 seconds off first placed man Tio Ellinas.

Sainz started in 15th place for race 2 on Sunday morning. At the end of lap 1, he was in 9th place. As they approached lap 2, the safety car was deployed due to an accident behind, giving Sainz time to save his tyres. With 5 laps remaining, Sainz was up to 8th place. During the next lap, he put a move on Jack Harvey's ART car to move up into 7th place. Sainz eventually would finish in 7th place. However, post race investigating revealed that Sainz's car was underweight, so he was disqualified from the final results from race 2, thus scoring no points. His teammate, Kvyat, retired during the race due to contact with another driver.