During a decade with the Welsh region, Thomas helped his side to two RaboDirect PRO12 titles and two second-place finishes. He also appeared in 52 Heineken Cup matches and scored 14 tries in all competitions.
The 6ft 5in, 17st 13lb flanker, who can also operate at Number Eight or lock, has also impressed on the international stage making 67 appearances for Wales, scoring seven tries.
Thomas broke into the Welsh set-up during the summer tour to Australia in 2003 and later played a starring role in the 2003 Rugby World Cup. His eye-catching performance against New Zealand in the final group game announced his talents to the wider rugby world.
The Pembroke-born ace was named in eight consecutive Six Nations squads between 2004 and 2011 and featured in the Grand Slam winning teams of 2005 and 2008.
Thomas was also selected for the 2007 Rugby World Cup and appeared against Canada before scoring against Australia.
Director of Rugby Dean Ryan described Thomas as a crucial signing for Warriors that would add vast experience at the highest level to the squad.
"We are delighted to secure a player of the quality of Jonathan Thomas," said Ryan. "He has huge experience from his time with the Ospreys and Wales and is versatile and can operate right across the back five.
"Jonathan is an extremely loyal and dedicated individual, underlined by the fact he has always been a one club man with Ospreys. However, a new challenge at Warriors at this stage in his career is one that excites him.
"He has experience of building from the start, having been at Ospreys since their inception in 2003, and he can now look to help us build a bright and successful future here at Sixways."