The young Italian will provide back-up and support for Marcus Ericsson and Felipe Nasr as the Swiss constructor look to vastly improve on their 2014 campaign, from which they failed to gain a single championship point.
Marciello has come through the Ferrari Driver Academy and is relishing the chance to prove himself at the highest level, hailing Sauber as the perfect place for him to begin his F1 career.
He said: "I am really pleased to become a member of the Sauber F1 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 want to thank the Ferrari Driver Academy for giving me this important opportunity, and now I really can't wait to start delivering my best.”
Team principal Monisha Kaltenborn has high hopes for Marciello - won the Formula 3 European Championship in 2013 before finishing eighth in his maiden GP2 Series this year - and revealed he will get the chance to drive the F1 car throughout the season in practice sessions.
"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," she said.
"We will give Raffaele the opportunity to become familiar with Formula One, which also includes him taking part in some free practice sessions on Friday.
"It's great for our team to be part of his way to Formula One."
Marciello will continue to drive in GP2 for Racing Engineering alongside his new role with Sauber.