"Having worked with players like Dwayne Peel and Mike Philips as they came through the youth system, Tavis has excited me as much as those players when they were 18, and shows the signs of an exceptional ability.
"Bringing him into the Scarlets, shows our ongoing commitment to grow an exciting and talented group of young Welsh players who will not only be future Scarlets stars, but we hope will be great prospects for Wales at senior international level.
"The latter part of our performance this season showed where our future is heading with a very promising group of players under 25 - many of whom have really stepped up this season and shown what they are capable of. As we rebuild this squad, we will combine the development of this talent alongside the experienced senior playing squad, which we believe will strengthen Scarlets rugby in the long-term."
On joining the Scarlets, Knoyle said: "This is a great opportunity for me and I am really excited about joining such a passionate club, which has its heart in the right place, has a great rugby tradition, so much talent and a bright future.
"I have enjoyed my season with Neath and would like to thank the club and its supporters for all they have done for me as I move on.
"The Scarlets have already proved they have got a lot of time for youngsters, they want to develop them and I'm really looking forward to learning from the coaches and the players around me. The Scarlets have got a lot of very experienced top players and it will be good to be part of this squad.
"I am very excited about my future with the Scarlets. I grew up watching them and have seen what incredible teams and players they have produced over the years. And now I'm putting on the jersey, its brilliant and I can see myself being really happy at Parc y Scarlets. I have come here for the rugby and I know I will get the opportunity to play here."