The 20-year-old made his GUINNESS PRO12 debut against Ulster last November before making five league appearances in total - including the 21-18 semi-final loss to Munster.
The lock also captained Wales U20s and his progress was recognised with a call-up to Warren Gatland's World Cup training squad.
It has been a rapid rise for the Swansea-born tyro and he admits that he still has to pinch himself in amazement at how quickly everything has happened.
"Coming through the ranks at the Ospreys, this has always been my first goal, to sign a professional contract, so I'm really happy to have done that," Thornton told the Ospreys official site.
"For someone of my age, it's a real achievement, something that I can be proud of. Looking back at the last year, it's been really incredible.
"Last summer I would have said that I was hoping to train with the senior team and get some LV games but it's been far better than I could have hoped for, playing in the PRO12 semi-final, captaining Wales and everything.
"Making my first PRO12 start up in Edinburgh was tough, it showed me how high the required standard is if I'm going to play regularly, but it also gave me a taste and I want to play more for the Ospreys.
"I know that I've got a lot of hard work ahead of me and that I can't take anything for granted.
"This is my home region so I get a buzz from being involved, training with people like Alun Wyn Jones, and getting some incredible experience that will hopefully allow me to kick on from here over the next few years."
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