Marciello, 20, already has prior experience of Formula One machinery, having tested for Ferrari in Abu Dhabi in November.
The Italian will combine his new Sauber duties with a second full season in GP2, in which he won once and claimed four podiums en route to finishing eighth in the 2014 championship.
“I am really pleased to become a member of the Sauber F1 Team, which has a long tradition of building up excellent drivers," Marciello said.
"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. 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.”
Sauber, who have signed Marcus Ericsson and Felipe Nasr as their race drivers for the 2015 season, said Marciello's role will include select Friday FP1 outings during the year.
“We are pleased to announce Raffaele as our test and reserve driver for the 2015 season," Sauber team principal Monisha Kaltenborn said. "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.
"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 has been a member of the Ferrari Driver Academy since 2011, when he scored two wins and six podiums in his rookie season in Italian Formula 3. He stepped up into the FIA European F3 championship the following season and was a title contender, before returning and storming to the title in 2013 - success which paved the way for his move into GP2 in 2014.