He should now stay at the club until the end of the 2016 season.
The hard-working back rower is now looking to help drive the Eagles towards their dream of reaching Super League.
“It’s good to get it wrapped up early in the season, knowing the next two years are all sorted,” Knowles said.
“It’s always good to stay at the same club for a few years. Especially at a club like Sheffield – Tubbs (coach Mark Aston) is a great man, it’s all setup properly, it’s a club to be around and a great set of lads.
“We’ve got the new stadium to look forward to next year and with the new league structure, I know Mark is quite ambitious and he is hoping to take us back up to Super League.
“We’re a few games into the season now – we’ve had a look at the teams like Bradford and London who have dropped down and we truly believe we can beat them on our day.
“Getting into that top four is our first priority and anything after that is a bonus.”
Eagles director of rugby and head coach Mark Aston is delighted that Knowles has extended his stay.
“He is one of those players that opposition players loathe playing against,” he said.
“He’s grubby, messy, nasty and everything else you want from good back rower, but he can play.
“He’s got the skills – kicking, passing, his knowledge of the game is good and he is a winner. He’s tough and that’s why other players don’t like him.
“We’re delighted – he has been with us for three years now and he has grown in stature. Hopefully there is even more to come from him.”