The all-action 23-year-old started his rugby career in his home of Glynneath and played for both Neath RFC and Llanelli RFC before making one appearance for the Ospreys.
However, it is at the Scarlets that Knoyle really made his name, developing into an international calibre player and first named in the Wales squad to tour New Zealand in summer 2010. He made his debut against the All Blacks in Dunedin in June 2010.
Knoyle - who has also played Wales Sevens - was named in the Welsh RBS 6 Nations squad for 2011, winning his second cap in the clash with Scotland at Murrayfield, and was part of the 2011 Rugby World Cup squad.
Director of Rugby Phil Davies said: "We are very lucky that we have got some very good scrum halves here in Lloyd Williams, Lewis Jones and young Tomos Williams coming though. But we need depth and Tavis is a little bit different; as a unit they will complement each other.
"Tavis is someone I've known from my Wales Under-20 days, where I gave him his cap, and he is a very hard working scrum half. He is a threat around the base with a good box kicking game.
"Tavis fits in as he is a guy who wants to be part of a team. He has good team values and a fantastic work ethic.
"It's a terrific opportunity for us and it's great for Tavis. I know he is delighted to be coming home and we are delighted to have him. We've signed an outstanding young Welsh player."