The former international outside-half is returning home to Wales following a disappointing season in France with Pau.
Thomas, 29, has struggled to gain any regular rugby since moving to France last summer amid a string of injuries but will now fill the gap left by the retiring Jenkins below current Wales squad member Ceri Sweeney.
'He will bring a lot of experience to the squad as well as being capable of playing some very good rugby,' said Warriors chief executive Leighton Samuel.
'We're giving him another opportunity to play at highest level in Wales and he's keen to take it.
'We needed an experienced outside-half to provide cover, particularly during the international period, and he fits the bill perfectly.'
Thomas is the fourth highest points scorer in Welsh rugby history having scored more tries (11) than any other Wales outside-half but it is nearly four years since he won the last of his 23 caps as a replacement against South Africa in 2000.
His return bucks the looming trend with France's biggest clubs trying to lure Wales' best players. Gareth Thomas is considering a lucrative offer from Toulouse while Stephen Jones and Shane Williams have also been targets for approaches.