The 32-year-old beat off competition from Wigan full-back and reigning Man of Steel Sam Tomkins, and Australians Cameron Smith, Ben Barba, Cooper Cronk and Nate Myles to claim the prestigious prize.
Sinfield told the Rhinos website: "It is a huge, huge honour. It was really nice just to be nominated, so to actually get the award is a bit surreal. I am really humbled by it."
Sinfield is only the fourth Englishman to take the honour, awarded annually by Rugby League World magazine, after Ellery Hanley (1988), Garry Schofield (1990) and Andy Farrell (2004).
He added: "You certainly have to pinch yourself. I would never have dreamt of being on a list with such illustrious Englishmen, but not only that, people like Wally Lewis....there are some absolute immortals of the game. It is such a huge honour and obviously to follow my hero Ellery, it has been really special.
"There were some fantastic players on the shortlist so to come out on top is really nice, but I think it is testament to the teams I have played in with Leeds and England. I've played amongst fantastic players."
Sinfield skippered Leeds to a sixth Super League success in nine years, famously overcoming a sickening blow to the head to be named man of the match in the Grand Final victory over Warrington.
The Rhinos also beat Manly Sea Eagles in the World Club Challenge and reached the Challenge Cup final under Sinfield's captaincy. In addition, he was named as the new captain of England and led the national side to victory in the 2012 autumn international series.
It later emerged he had played throughout the year with a fractured wrist, on which he has now had surgery. Yet Sinfield does not want to take all the credit for his success.
He said: "I think this club and this team is all about team awards. It is really nice to win an individual one but without every one of my team-mates and every one of the coaching staff, and everyone else doing their jobs, and doing them very well, it wouldn't allow me to do my job. We all play a small part in the organisation and I think this award represents the team, not just myself."
- Sports & Recreation
- Ellery Hanley