The ex Swansea star will return to Wales following a spell playing in France with Pau.
He will compete with Gareth Bowen for the number 10 shirt at Stradey Park, following Stephen Jones' decision to leave Llanelli for Montferrand.
Thomas, 29, had signed a deal with the Celtic Warriors before they were disbanded.
'I want to give it a good crack and get my form back,' Thomas told the South Wales Evening Post. ''I believe I have four good seasons left.'
Thomas' move comes on the day that another former Warrior, Tongan flanker Maama Molitika, has been signed by Harlequins.
Warriors and Wales full-back Kevin Morgan, one of the biggest names left searching for a contract, says he is confident of a deal for next season.
Morgan says he is in talks with the four remaining Welsh regions, but that he could be forced to take up offers from outside the country.