Sainz jr is also sponsored by Cepsa, the Spanish oil company and sponsor of Red Bull's second formula one team Toro Rosso.
Last year, he raced in the British and European F3 categories.
"I am seeking more consistency," said Sainz jr, "because GP3 is a very serious thing, with laptimes only six seconds behind formula one.
"Christian Horner has the ultimate responsibility, so I'll be even more linked to Red Bull," he added, revealing that he is moving to live in Milton Keynes.
"It's a whole new experience and I really want to get started."