Thomas, 30, who joined Gloucester Rugby ahead of the 2014-15 season from the Scarlets, will return to the region that he first represented in 2004.
A former 7s specialist and 2009 IRB Sevens World Cup winner with Wales - scoring the winning try in the final against Argentina - Thomas stepped up to the pivotal role at the Scarlets whilst international fly half Rhys Priestland was out with a long-term Achilles injury in 2013-14 and helped guide the region into the top four and semi-finals of the PRO12.
Thomas, a valuable and respected senior player who had established a strong goal-kicking percentage at the Scarlets, originally from Bancyfelin, Carmarthenshire, has been released from his contract with Gloucester Rugby to make a return to West Wales.
Scarlets Head Coach Wayne Pivac said: "We are pleased to be welcoming Aled back to the Scarlets. He's an experienced local player who knows what it takes to represent this region and will offer support to Steve (Shingler) at the fly half position.
"Having a player of Aled's experience and maturity within the squad will have a positive impact and will help young players like Josh (Lewis) and Dan (Jones) to further develop their game."
Speaking about his return to the Scarlets Thomas said: "I am hugely excited to be rejoining my home region. I'm very grateful to Gloucester Rugby who have agreed my release so that I can return to the Scarlets where I have previously enjoyed three fantastic seasons.
"The prospect of rebuilding successful playing partnerships and playing in front of the Scarlets faithful and vocal supporters is something I look forward to."
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