The Dutchman, who beat ex-GP2 racers Jules Bianchi and Sam Bird to the FR3.5 title after stepping up from Formula Renault 2.0, spent a day driving for the team at the recent Abu Dhabi young driver test.
With Esteban Gutierrez moving up from reserve driver to a race seat for 2013, Sauber confirmed on Friday that Frijns has joined its line-up.
Team principal Monisha Kaltenborn said: "Monitoring Robin's racing career makes it easy to spot his potential.
"This was the reason we gave him the chance to drive the C31 in Abu Dhabi. He managed this very well.
"We will now carefully guide him to Formula 1. This is a long way, but Robin has got the skills to do that successfully."
Frijns, who has won three junior championships in succession (FR3.5, FR Eurocup, Formula BMW Europe), had been expected to land a deal with Sauber if he performed well in the Abu Dhabi test.
"I'm very happy the Sauber F1 team has given me this opportunity," he said. "I would like to thank Monisha Kaltenborn and Peter Sauber for their belief and trust in me.
"With this opportunity I will try to help the team as much as possible and get the chance to learn how Formula 1 works in an extremely professional environment.
"I am already very much looking forward to getting the 2013 season started and working together with the team."
Robin Frijns CV
2012: Formula Renault 3.5 champion (3 wins)
2011: Formula Renault 2.0 Eurocup champion (5 wins), fourth in Formula Renault 2.0 NEC (part-season, 1 win)
2010: Formula BMW Europe champion (6 wins), three starts in Formula Renault 2.0 NEC (1 win)
2009: Third in Formula BMW Europe and Rookie Cup winner (1 win)