The 30-year-old has played 188 games for Ospreys - winning four league titles and an EDF Energy Cup at Twickenham - and 67 times for Wales.
But he now makes the move to England for a first crack at the Aviva Premiership with Warriors and admits a new challenge and the chance to mix it in the English competition is a huge draw.
Thomas said: "I have been at the Ospreys for a long time and I'm grateful to the supporters, coaches, players and management for all the backing they've given me. This a massive new opportunity for me.
"There was some interest from other clubs, yet when I spoke to Warriors I knew this was the way forward for me.
"The Aviva Premiership is very exciting," he added. "I've got mates with various clubs in the league and they always talk about the intensity of the competition.
"I'm a fan of rugby and love watching it. I've always been interested in the Aviva Premiership and it is one of the great competitions in world rugby.
"The sheer intensity week in and week out is huge and the fact you have the play-offs and relegation means every game is massive. I have always had an ambition to be part of it and it's nice to have a fresh start.
"I have a massive respect for the Ospreys and the competition I have been a part in my rugby career. But it will be great to test yourself at new venues, against new clubs and new players every week."
Thomas, part of the Wales Grand Slam winning teams of 2005 and 2008, has always been a one-club man after being with the Ospreys since inception in 2003.
He acknowledges the move to a new club will be a completely new experience for him.
However, some familiar faces from the past have already given Warriors a glowing recommendation and he is determined to jump into life in England with both feet.
"There are a few guys I know in the squad, but it will be a very strange experience starting at a new club after all this time with one team," admitted Thomas.
"I came up to the Warriors for a medical and joked with the medics that I had never had one before, it was a first for me!
"I have chatted with some people I know to get their opinions. I spoke to Ed (Shervington) and Nikki (Walker), who both go back to our Ospreys days, and both talked very highly of the club and the players there. That was good to know, that good people are here to make us a success.
"It's a busy but exciting time," added Thomas. "My second child is due in July, but we are moving up to the area as soon as possible because I want to get involved with the club and the area as much as I possibly can.
"Sometimes when you commute you can't get fully integrated. I plan to jump straight into my new club and environment and I'm really looking forward to pre-season and becoming a Warrior."