26-year-old Thomas, the cousin of fellow front-row Gareth, will join up with his new teammates at the end of the campaign.
A product of Newcastle Emlyn RFC, Thomas has made 51 appearances since breaking into the senior setup for the Scarlets in 2017.
The 6’1” loose head has also played for Carmarthen Quins and represented Wales at both under-18 and under-20 level.
Speaking on his move, Thomas said:
“It’s great to be joining the Ospreys next season. I know a few of the boys down there, including Gareth, and they’ve all spoken about the positive atmosphere at the club.
“They’ve obviously got some quality, experienced internationals in their setup, but outside of that, there are some really promising youngsters coming through which bodes well for the future.”
Head Coach of the Ospreys, Toby Booth added:
“Steffan has shown a strong competitive edge with a want to fulfil his potential. We have a strong emphasis on scrum and lineout and Duncan Jones will undoubtedly be looking forward to developing him in these areas.”
[Lead image Scarlets Rugby]
