Keinhorst scored seven tries in 28 appearances for KR this year, his debut campaign after moving from Leeds on a three-year deal.
However, he was told he could look for playing opportunities elsewhere for 2020 by coach Tony Smith who has brought former New Zealand star Shaun Kenny-Dowall and Castleford Tigers’ Greg Minikin in as further centre options for next term.
Leeds-born Keinhorst, 29, won the 2015 Grand Final with Rhinos but will now prepare to play in the Championship next season.
He joins fellow KR outside-back Elliot Wallis at ambitious York who continue to strengthen their squad after the significant signings of Hull FC utility Danny Washbrook, another former Rhinos Grand Final winner Chris Clarkson and ex-Rovers and Castleford prop James Green.
Smith said: “Jimmy will head to York during 2020 in search of regular game-time.
“He has continued to apply himself in pre-season training to date and we wish him well, alongside Elliot, with the City Knights across the next 12 months.”