Often derided for their poor finals record, Cronulla overcome an early deficit and then hung on late to claim victory in a Saturday night bellringer.
In doing so they ended the run of a red-hot Roosters side, who had come into the finals threatening to go deep as one of the form teams of the competition.
Cronulla will now travel to face the loser of Canberra and Brisbane next weekend, in a do-or-die semi-final.
The victory marked just Cronulla's second finals win in their past 10 outings, a run that stretches all the way back to 2018.
But after finishing the season in fifth there were few signs of finals jitters on Saturday night, in a match that lived up to the billing.
Ronaldo Mulitalo scored twice to win his highly anticipated bout with Mark Nawaqanitawase, with the latter also crossing once early in the match.
Braydon Trindall was superb in the Sharks halves as he set up both of Mulitalo's tries, while Sam Walker had plenty of good touches for the Roosters.
But ultimately the difference was Cronulla were able to make the most of their chances, as both sides had long patches of territorial dominance.
After Nawaqanitawase stepped outside Mulitalo for the Roosters' first try, it was a double blow from the Sharks winger that put Cronulla back on top.
With the Roosters having controlled the majority of the first half, Trindall sent Ronaldo over for Cronulla's first in the 35th minute when Nawaqanitawase rushed out of the line.
The Sharks were then in again moments later, when Walker had a ball knocked out of his hand and it flew backwards off his foot to allow Cronulla to go on the attack.
With the Roosters' defence scrambling, it was Trindall again who sent Mulitalo over with a loopy cut-out ball for the winger to make it 12-4 at the break.
And while a smart Walker grubber for Angus Crichton got the Roosters back within two shortly after half time, the Tricolours couldn't convert any more chances.
Instead it was in-form hooker Blayke Brailey who sealed it for Cronulla, sending Toby Rudolf over next to the posts in the final minutes.
For the Roosters it ended a season where few expected them to play finals after conceding 50 in the opening round and sat in the bottom four for the opening month following last year's mass exodus of experienced players.
But their season swung with the unearthing of several young stars, Walker's return from a ruptured ACL and Nawaqanitawase's theatrics in his first full season from rugby union.
And while there were signs of all that on Saturday night, they ultimately left themselves with too much to do after finishing the season in eighth.
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