The scrum-half was named the Robins' Young Player of the Year for the 2016 season although he could not help preserve their top-flight status, as they were dramatically beaten 19-18 by Salford Red Devils in the Million Pound Game earlier this month.
KR will now compete in the Kingstone Press Championship next season but, despite their relegation, Marsh has become the latest player to commit his future to Tim Sheens' team, honouring the long-term contract he signed with the club earlier this year.
The Robins' head of rugby Jamie Peacock admits Marsh's decision to remain at the KC Lightstream Stadium is a huge boost, and he confirmed the club will be looking to build their squad around the youngster going forward.
Peacock said: "He's young, hungry and improving all the time in one of the most difficult positions to play in the early stages of your career."
Peacock believes it is important KR place great emphasis on youth next season, as they bid to make an instant return to Super League.