Thomas, from Newcastle Emlyn, will join up with his new team-mates at Llandarcy Academy of Sport for pre-season ahead of the 2013-14 campaign. He becomes the fifth new face confirmed for next season after Rynier Bernardo, Josh Matavesi, Dan Evans and Sam Parry.
Thomas has been a consistent performer for Carmarthen Quins in the Principality Premiership over the last three seasons, making 46 appearances and scoring one try, including 20 outings in the current campaign. He also played in all six games in this season's British & Irish Cup for Llanelli, as well featuring once for the Scarlets.
Capped at age-grade level, Thomas was an integral part of a successful Wales U20 squad that reached the Junior World Championship semi-final in 2012 before going one better a year a later and reaching the final for the first time.
Acknowledging the size of the task ahead of him if he is to establish himself as an Osprey, Thomas said he can't wait to get started.
"I'm really happy to be joining the Ospreys, it's a great opportunity for me" he said.
"The Ospreys are the most successful region in Wales and there are lots of quality front row players in the squad already so I know that the standard required is high. There's real strength in depth there and a great development track record so it's a good place to be as a young player learning the game.
"I've got a three year contract and I'm looking forward to the challenge of establishing myself with the region. It's going to take a lot of hard work, but being part of the Ospreys environment for the next three seasons is a pretty exciting prospect. Hopefully I'll be able to do enough to impress the coaches and earn a place in the team."
Rugby Operations Manager, Andy Lloyd, said Thomas has earned his chance with the Ospreys after impressing with his club form.
"We've watched Gareth a lot with Carmarthen over the last few seasons and really liked what we've seen in him" said Lloyd.
"We look at a considerable amount of Premiership rugby and he has been a standout performer. When you add the quality of his work within the national set-up at U20 level, it was clear to us that he is a real prospect who will bring something positive to our environment and to the team.
"As an organisation we have a strong and successful history of developing players, and the front row has been a particular strength of ours. The recruitment of Gareth was a strategic decision, with that in mind, to allow us to maintain that record.
"He is a great character, someone who wants to improve and become a better player, and him coming into our environment will be beneficial for both parties. He's ambitious and wants to push our other loose heads, so there will be real competition for that spot over the next few years which is exactly what we want.
"He'd adding to our stocks of exciting young loose heads. Even though it's a young group of players we have in that position, we believe we are developing real strength in depth, and Gareth will only enhance that. If we have players pushing each other day in, day out, they will drive each other to become better, raising standards, and that will obviously benefit the team."
In addition to the five new signings for next season, since the start of 2014 the Ospreys have confirmed new contracts for 12 players. Skipper Alun Wyn Jones, Sam Lewis, Ashley Beck, Eli Walker, Jonathan Spratt, Ben John, Ryan Bevington, Dmitri Arhip, Nicky Smith, Dan Baker, Dan Suter and Lloyd Peers have all committed their futures to the region.
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