An imposing 6ft 6in and 18st figure, Day is an athletic line-out operator as well as being very effective in the loose and he will further strengthen Bath's second row resources for next season.
Sir Ian McGeechan, Director of Rugby, commented: "Dominic is already an experienced player, having played in both the RaboDirect PRO12 and the Heineken Cup. He has a good range of skills necessary in a lock, and at 26 has the potential to develop further. He has a great attitude and we have no doubt that he'll fit in very well with the Bath culture and be a great addition to the squad."
Day added: "Bath are a hugely ambitious club, and I'm very excited to be joining them. I've loved my time with the Scarlets, but feel it's the right time in my career for a change and to move on."