The Newport born player is a versatile back row, who can also play second row, will help bolster the forwards pack for the 2014/15 season.
The 23-year-old has played for Gwent sides Ebbw Vale, Pontypool, Newport and moved to Bedwas in 2012. He was appointed the skipper of Bedwas for the 2013/14 season.
Thomas who has honours from Wales with the U18s and the U20s has just started pre-season training alongside the men of Gwent.
Director of rugby, Lyn Jones commented: “I have always been a fan of James’ abilities.
“When I helped out as a coach in Ebbw Vale a couple of years back he stood out to me then as he had a great U20s campaign with Wales.
“Unfortunately James sustained a couple of injuries, during that period, but he came back and proved himself to be Bedwas’ best player over the two seasons he was there.
“I think that he will prove to be a very strong signing for Gwent as he progresses his talents throughout the season.”
