Speaking about his move, the Pembroke born forward said: "The Ospreys have been a huge part of my life for so long now that it is going to be really strange reporting for pre-season training somewhere other than Llandarcy. I'm really proud of everything we've achieved over the last 10 years, particularly winning four league titles and an EDF Energy Cup at Twickenham.
"What we've done at the Ospreys has also allowed me to represent Wales 67 times and I consider myself incredibly fortunate to have had those experiences.
"I'm grateful to the supporters, coaches, players and management at the Ospreys for all the backing they've given me. I've made friends who will stay friends for life whatever happens next. I've got some great memories to take away with me, going all the way back to my days as a teenager with Swansea.
"While sad to be leaving the Ospreys, I'm really excited about the future and relishing the prospect of experiencing a new culture and playing in a new competition, although I will still be following the region from afar."
Thomas leaves the Ospreys after joining Andrew Bishop at the top of the region's all-time appearance list on 188 in the defeat against Glasgow last month.
Having risen to attention as a teenager at Swansea, captaining the All Whites at the age of just 19, Thomas was part of the inaugural Ospreys squad in 2003. He made his debut against Leinster that November and just two months later, captained the region for the first time in a Heineken Cup tie against Edinburgh less than four weeks after his 21st birthday. The appearance made him the youngest ever Ospreys captain, a record that still stands.
Andrew Hore, Chief Operations Officer at the Ospreys, added: "Jonathan has been a tremendous servant for both the Ospreys and Swansea RFC over the last 12 years and we wish him all the best at his new club.
"There will always be a welcome for him back at the Ospreys in some capacity, be that as a player, administrator or coach."