The 35-year-old switched codes from league to join union side Yorkshire Carnegie last November after leading the Rhinos to the treble.
He captained Leeds for 13 years and won seven Grand Finals, three World Club Challenges and two Challenge Cups.
“I have always said that some day I will need to get a proper job and that day has now come,” Sinfield said.
The former England skipper, will retire after Carnegie’s Championship play-off campaign.
Bryan Redpath’s side face Doncaster Knights in a two-legged tie with the winner to play Bristol or Bedford for a place in the Premiership.
Sinfield, capped 27 times by his country in the 13-man code, said: “I have been privileged and deeply honoured to have enjoyed a 20-year career as a professional rugby player and to retire, on my own terms rather than through injury, is something I feel lucky to be able to do.
“I have thoroughly enjoyed the last seven months with Yorkshire Carnegie and I would like to thank the coaches, players and supporters for making me feel so welcome.
“I have always gave total commitment to every team I have ever played in and therefore I have decided to hang up my boots at the end of this season.”
Sinfield made his Championship debut for Carnegie in November and has scored 117 points in 16 appearances in all competitions.
He admitted last month that he would miss playing rugby league “every day for the rest of my life” and that he had under-estimated the differences between the codes.
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