The 30-year-old first represented them back in 2004 after coming through the Scarlets academy and has also enjoyed spells with Newport Gwent Dragons and London Welsh during his career.
However, he will now be seeking to add to his 69 PRO12 appearances and help the Scarlets improve on their sixth-place finish last season.
"I'm really looking forward and really excited to get back into it," he told the club website. "It was a frustrating year last year and I'm now looking forward to getting back into some rugby.
"I'll be working hard to try and achieve and try and play as much as possible. I'm really pleased to be back. I had mixed emotions leaving last season and I'm really happy to get the opportunity to come back.
"I'm going to try and make the most of it, play as much as possible, and hopefully make a big contribution to a successful season.
"There a lot of familiar faces here and some new boys too. They've made some real quality signings and I'm sure they'll make a big difference.
"There's a good core squad here for the start of the season which will hopefully put us in a good position before the internationals return and will see us pushing for the top four."
Wales fly-half Rhys Priestland left the Scarlets for Bath at the end of last season but Thomas still faces stiff competition for places, with Steve Shingler also jostling for the No.10 shirt.
"There's always pressure whatever position you're in," he added. "There's pressure in every game and every training session.
"I try not to think of people who've played in my position previously, it's important to think in the present and perform on the weekend.
"Steve (Shingler) is a quality player, there are a number of good outside halves here. Steve took his opportunity last year and played well, there's plenty of competition, there always is.
"I'll be working as hard as possible to push for my place and push for the jersey."
Follow us on Facebook, join the conversation on Twitter, sign up to our YouTube channel for extensive match highlights and sign up for our newsletter for regular updates on the GUINNESS PRO12
63x46.png){kind=logo}