The second row's Scarlets contract comes to an end in May 2012 and has not been extended by the region into 2012/13 as part of their planning for the season ahead.
English qualified Day, 26, signs for the Aviva Premiership side Bath for three years after five seasons with the Scarlets and 97 appearances since joining the senior squad in 2006/7 season - scoring 15 points in his Scarlets career.
He joined the West Wales region in 2004 and after being developed as an academy player worked his way through the Scarlets system to the first team where he has been a regular as well as playing for Llanelli RFC.
Following an announcement made by Bath Rugby late last night, the Scarlets would like to wish the popular West Wales player their best wishes as he looks to the opportunities provided in the English Premiership.
Scarlets Head of Rugby, Nigel Davies said: "Dom has been a good servant to the Scarlets for a number of seasons and now at 26 and at a key time in his playing career feels it's time to gain some different playing experience in the English league.
"As an English qualified player as well as Welsh, a move to Bath also puts Dom in a place where he believes he can in the future push for potential international aspirations beyond club rugby.
"He has been an athletic player and a great character to have around in our environment, he has much to offer Bath and I am sure will progress forward well with them. He leaves with our best regards and we all wish him the very best in his future career and we thank him for his contribution and commitment to Scarlets rugby."
The Scarlets Head of Rugby added:"We have been reviewing our squad make-up for next season and this is a process that we invest a lot of our time and thought into. We will make any announcements about recruitment or contract extensions and conclusions in a structured, timely way that respects our values and the primary responsibility we have to our individual players, our squad and our rugby environment as a whole.
"Our focus on development within the region and investment in emerging players of the future plus the recruitment of any new players will reflect our vision as Scarlets and future ambitions as a competitive and sustainable rugby region and business.
"Development and recruitment into our squad next season will be in the context our five-year business plan and continues our focus on providing opportunities for the development of young rugby talent for Wales as well as maintaining core experience and building strength-in-depth."
Pembrokeshire based 6ft 6in Dominic Day, from Lamphey, thanked the Scarlets for the development and support they have given him in his rugby career and for the opportunities he has had at Parc y Scarlets.
He said: "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. I've gained experience and progressed with the Scarlets through the opportunities handed to me in both the league and Heineken Cup and that has really progressed me as a player in recent seasons. The Aviva Premiership is a hugely competitive league and playing in it is a challenge I've always wanted, so I'm really looking forward to getting involved and pushing myself in a new league.
"I would like to thank all the Scarlets supporters in West Wales for backing me over the years and it's been a big honour and source of pride for me to fly the flag for Pembrokeshire at the Scarlets.
Day who has featured in 20 fixtures for the Scarlets this season said: "We have important games in our league campaign tonight against Treviso at home and then Connacht at Parc y Scarlets next week - and that's my primary focus and I'll be giving my all to the rest of our campaign this season out of respect for the jersey and my fellow Scarlets."
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland