Sherwin, 32, has made 52 appearances for the Tigers since arriving from Canterbury at the beginning of 2008, and moves to France to fill the void left by the departure of Adam Mogg earlier in the season.
Tigers CEO Richard Wright said: “Brent’s contract with us was at its end at the close of the current season and although he would have liked to have stayed he could see the positives in this move. He wasn’t in our plans for 2011, so the timing was right and this move will give him a chance to stake his claim for a longer deal with the Dragons.
“He has put in some match winning performances for us and has played at times through injury but we are looking to make changes for next season and we couldn’t have stood in his way when this opportunity arose.”
Castleford have moved quickly to secure the services of Huddersfield half-back Kyle Wood on a month-loan deal.
Wood, 20, has been playing for Batley on a dual-registration contract after moving to the Giants in the close season, having impressed last year playing for Sheffield.
Wright added: “We have brought in an immediate replacement in Kyle Wood and we will be exploring other options over the next couple of weeks. Kyle is a highly regarded, exiting young player who is keen to make his mark in Super League.”