"Ben joins us with great experience and leadership having been a regular performer with Waikato Chiefs and this season with Western Force in the Super 14 competition, in addition to his time in France," said Dragons Head Coach Paul Turner.
"He has good leadership skills having captained Bay of Plenty and is also familiar with our captain Tom Willis from their Chiefs days. He will be a solid addition in the make-up of the Dragons squad."
The former captain of New Zealand provincial side Bay of Plenty won 42 Super Rugby caps with the Chiefs during his five seasons in New Zealand after debuting against the Crusaders in 2004 and is currently representing the Western Force as their marquee international player in the Super 14. He also represented the Junior All Blacks against Australia A at the Sydney Football Stadium in 2005.
"I am very pleased to be joining Newport Gwent Dragons," he said. "I am looking forward to joining up with Tom Willis again, who I played alongside in the Chiefs for five years and after playing against Hoani MacDonald numerous times, I am pleased to be playing with him.
"I am excited about the direction of the region. There is a great base of players there and I am really looking forward to the season and its successes."
Castle (29) will replace Dragons tight head prop Rhys Thomas who joins to the Scarlets at the end of the season.
Thomas, who picked up his seventh Welsh cap against Italy in the 2009 Six Nations has been with the Dragons since the inception of regional rugby and has notched up 98 starts for the Men of Gwent.
"I've enjoyed my eight years with the Club, having come through the youth structure," he said. "I've made some very good friends here. It's a great squad of players and I wish them all the best. I'm particularly grateful to the fans who have been really good to me during my time at Rodney Parade."
Paul Turner added "Rhys has been a tremendous servant to the Dragons and the club wish him the best of luck in his new challenge."
Dragons Chief Executive Gethin Jenkins said "We would like to thank Rhys for his service to both Newport and the Dragons and wish him well down west.
"Whilst we have lost Rhys, we are confident that we have recruited someone able to replace him in Ben. He will bring with him a tremendous amount of leadership and experience to our exciting blend of emerging Welsh and international talent."