He captained the side in Warriors' opening pre-season friendly victory against Rotherham Titans and led the side again on Saturday for the visit to his former club.
Warriors Director of Rugby Dean Ryan said: "Jonathan has been an asset to Warriors since he joined us in the summer, both in his performances on the pitch and also in the qualities he demonstrates off the pitch.
"He's a strong leader who understands the game and his past experience with Wales and Ospreys will be important in building the future here at Sixways."
Vastly experienced back row Thomas will make his competitive Warriors debut when the season gets underway at Leicester Tigers next month.
During a decade at Ospreys, the 30-year-old helped his side to two RaboDirect PRO12 titles and two second-place finishes. He played 188 matches for the club - including 52 Heineken Cup appearances.
The 6ft 5in flanker, who can also operate at number eight or lock, has impressed on the international stage - winning 67 caps for Wales.
He 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.