Knoyle will move to the Arms Park from Aviva Premiership club Gloucester at the end of this season.
Knoyle, 23, has won 11 caps, making his debut against New Zealand in 2010, but he is not part of Wales' current RBS 6 Nations squad.
" 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," Blues rugby director Phil Davies said.
" 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.
"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."
- Sports & Recreation
- Aviva Premiership