"We are delighted that Martyn has joined us," commented Rugby Manager Jim McCreedy. "He is a very promising youngster who we believe has a big future ahead of him.
"He fits in with the region's philosophy of continuing to develop youngsters and already we have had success stories with Ashley Smith, Jamie Corsi, Alex Walker and Aled Brew all establishing themselves in the Dragons squad.
"Martyn is an exciting young talent and a welcome addition to the squad, we hope he will follow in the footsteps of Aled Brew onto the international stage."
Thomas started for Wales in last weekend's match against Ireland U20 and has been selected to face Scotland U20 this week.