The hard-hitting Knights forward, who can play in both the middle and on the edge, is currently serving a six-week suspension at the Knights and is due to return in Round 10 after a stray elbow which was directed at Chris Smith during a Round 3 clash against the Penrith Panthers.
Barnett has played 117 NRL games, with 115 of them at the Knights after playing two games for the Canberra Raiders during his debut season.
The move to Auckland will give the forward a chance at a fresh start, and Warriors general manager of football Craig Hodges said Barnett will bring plenty to the pack.
“We couldn’t be happier to have a player of Mitch’s proven ability on board from next year,” Hodges said in a club statement.
“He has been on our target list for some time as we look to boost our pack. He brings a level of intensity and passion which is going to be a real asset for us.”
It likely means he will go straight into the starting forward pack at the Warriors next season, with Euan Aitken to vacate a second row position after his request for a release.
The Knights, on the other hand, will go into the market for a replacement in the forward pack, which the club currently having made no signings for 2023.
Barnett said he was looking forward to further his game at the Warriors.
“The Warriors presented me with an opportunity to further my game,” Barnett said.
“I have personal goals of playing representative football and I am also looking forward to working with Brownie again.
“The Warriors impressed me on and off the field which made my decision easier.”
Barnett has previously played under head coach Nathan Brown at the Knights, while the speed of the deal may come as a surprise, with the Warriors only spending five weeks at the negotiating table with Barnett.
“Once we were given permission to negotiate with Mitch five weeks ago, we couldn’t wait to get him across the line,” Warriors CEO Cameron George said.
The Warriors have made a strong recruitment push for 2023 already, with Barnett their fourth signing following Marata Niukore from the Parramatta Eels, Dylan Walker from the Manly Sea Eagles and Luke Metcalf from the Cronulla Sharks.