The 20-year-old won the European Formula Three Championship in 2013 and came eighth in the 2014 GP2 Series.
"He proved his great talent in junior formulas, and his season in the European Formula 3 Championship with 13 wins and the title victory was particularly impressive," said Sauber team principal Monisha Kaltenborn.
Marciello, part of Ferrari's driver academy, added: "I am really pleased to become a member of the Sauber Formula One team, which has a long tradition of building up excellent drivers.
"I really think that this is the best choice for my future career. I am also happy to compete for one more season in the GP2 Series, which, together with my new F1 role, will give me a very busy 2015 season."