The England captain, 34, has signed an 18-month contract with Yorkshire Carnegie and will join the squad at the end of the Super League season.
Sinfield has skippered Leeds since 2002 and has won three World Club Challenge trophies, six Super Leagues, two League Leader’s Shields and the 2014 Challenge Cup.
He boasts 14 Test caps for Great Britain and last year was made an MBE.
Speaking of his decision to switch codes, Sinfield said: "My entire adult life has been spent at Leeds Rhinos and I am extremely grateful for all the support I have had over the past two decades.
"This was one of the toughest decisions of my career but I know the time is right for me to finish my time in rugby league and embark on a new challenge with Yorkshire Carnegie.
"I never wanted to play against the Rhinos and throughout my career I have always had an admiration for rugby union and wanted to challenge myself in that code.
"I am fortunate as a professional player in this era that opportunities like this exist for me and it is something I want to grab with both hands."
The Oldham-born forward joins Sam Burgess as another big-name league player to switch codes in recent months.
Burgess signed for Bath after ending his South Sydney Rabbitoh’s career with the Premiership title last season.