Gallacher said: "This has been a hard decision for me to make after so many years connected with Llanelli as a club and region and is something that I have been considering for several months.
"I wanted to see the Scarlets through the move from Stradey Park to the fantastic new home at Parc y Scarlets, and now we have had our official opening and firmly settled into our operations here at the new stadium, I feel the time is right to retire as Chief Executive and move on at the end of the season.
"I am very grateful to all those who have supported me during my time as Chief Executive and it has been a privilege to serve this great rugby region for so many years. My thanks also goes to supporters of this region who remain loyal and passionate about the Scarlets and I look forward to seeing them enjoy the success they deserve for years to come as supporters of this region.
"We are lucky to have a very strong and highly experienced team of funding directors - each of whom are passionate about the Scarlets and are committed to the region's long-term success. I am sure that they will appoint a successor who has all the qualities that this exciting and challenging role requires and ensure that the Scarlets capitalise fully on the potential of this new stadium as one of the finest rugby facilities in Europe."
The region will also bid farewell to its Managing Director, Gareth Davies who leaves at the end of the season to take up a new role as Business Development Director at UK telecoms leader, Link Telecom.
An interim Chief Executive, Ian Kirby, has been appointed by the Scarlets board to oversee the transition of the departures of Stuart Gallacher and Gareth Davies and the arrival of a new Chief Executive to take over at the region.
Chairman of the Scarlets board Huw Evans said: "We are grateful to Stuart and Gareth for the significant contributions each of them has made to the region and particularly for their hard work and commitment managing the move from Stradey Park to Parc y Scarlets.
"I would like to play particular tribute to Stuart for his dedication to the Scarlets for so many years. Without his tireless commitment and support of this region at the most difficult and challenging of times, it is highly likely that the Scarlets would not have survived. He played an important part in winning two public inquiries to allow us to move from Stradey and a massive role in our bid to stand alone as a region back in 2003. He has also played a crucial role at the top of the European rugby scene on the ERC board - acting as a driving force for the interests of Welsh rugby.
"His determination, passion and belief in the Scarlets is something that we have been very grateful for and lucky to have had at key times in our history and development.
"This is a time of great opportunity and equally great challenge for the Scarlets as we build on the potential that the new stadium offers this region. There is plenty of work to be done and the spotlight is on the Scarlets' commercial performance, regional development and the team's results on the field as never before. The funding directors and I are committed to appointing a new Chief Executive that will harness the strength and tradition of this region and the search is already underway."