The 23-year-old actor, who played Weatherfield's Adam Barlow from 2004 to 2007, has signed up for the role of art history student Innes Maitland on the BBC One show.
Robertson's character will immediately attract romantic attention from Amber Murdoch (Lorna Anderson) when he hits screens later this year.
Discussing his new role, he revealed: "I'm having an amazing time filming River City. Innes is a very cool, confident character and he's a lot of fun to play."
Meanwhile, TV newcomers Ryan Smith and Lorna Craig have been cast as siblings Charlie and Jennifer Bowie, who share a flat with their fellow student Innes.
River City's executive producer Gaynor Holmes said: "It's wonderfully exciting to welcome a mix of established actors and fresh new talent to River City.
"The arrival of the students will bring a lot of fun to the drama. Samuel will already be recognisable to many of our viewers, and it's a pleasure to also introduce talented newcomers Ryan Smith and Lorna Craig."
The trio will make their first appearances on the programme in June.