Already signed until the end of next year, Cronulla made the call to avoid any risk of Hynes going to the open market with a lengthy extension worth well beyond his current $600,000 salary.
The contract means Hynes will be 33 by the time the deal ends, all but ensuring he will finish his career in Sharks colours after moving from Melbourne at the start of last year.
A utility player at the Storm, Hynes became one of the most dangerous playmakers in the game last season on his way to polling the most Dally M votes in history with 38.
In a statement on their website, Cronulla labelled the news as "one of the most significant signing announcements in Sharks history".
It follows the long-term extension taken up by Craig Fitzgibbon last month, with the coach signed on until the end of 2027 after taking the club to the finals in his rookie season last year.
"Nicho has chosen to sign this extension without going to market, with a view to keeping this team together," chairman Steve Mace said.
"But it is also an indication that he believes in what the club is building on and off the field.
"Having recently extended Craig as well was a big contributor to his decision.
"To now have Nicho and the coach on board long term is a huge positive for the NRL team and our club."
Hynes' deal makes him the longest-signed player in the NRL, with no other player locked in to any club beyond the end of 2028.
It comes after he had to wait until age 23 to debut for Melbourne, after initially going unsigned by Manly following under-20s.
Hynes forced his way back into the NRL by impressing in the Queensland Cup and becoming a super sub at the Storm, before becoming a star in his own right in his first year at the Sharks.
"Loyalty is a big value of mine, and this club and Fitzy gave me an opportunity to be a starting player," Hynes said.
"It's pretty surreal, it hasn't sunk in yet. I never thought I'd sign a long-term contract in the NRL, one that's hopefully going to set up my life and my family as well.
"I'm happy it's done and now I can focus on playing some good footy and to win some games with this club, with the ultimate goal of winning a premiership."
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