The 42-year-old, who is the longest serving Welsh regional coach, has worked at elite level in rugby for the past 15 years, was formerly with the Ospreys since their inception in 2003 initially as assistant coach and then in 2008 when he was appointed as Head Coach where he remained for three years.
During that time Holley lead the Ospreys to notable victories including Magners League Champions in 2004/5, 2006/7, 2009/10 and 2011/12 and Heineken Cup quarter finalists three years in a row in 2007/8, 2008/9 and 2009/10.
Scarlets Head Coach Simon Easterby said it was a positive for his team to have Holley's knowledge and experience of the regional game.Easterby said: "To have a coach of Sean's experience, expertise and technical ability at a key time for us is vital as Mark goes away on Wales duty.
"Mark Jones has contributed a great deal to our rugby environment and team this season and deserves his opportunity with Wales and we are pleased to support the national set-up and his coaching development. Sean will be looking to continue the good work Mark has already put in place.
"We have a crucial period ahead of us while nine of our senior players are away during the Six Nations and need to make significant progress in the RaboDirect PRO12.
"We are fortunate, given the time-frame and midway through the season, to have secured the services of someone of Sean's capability and background.
"He will also bring a fresh new perspective to our team environment which will be healthy for our coaching group and our squad of players.
"We look forward to his input; he will challenge and support our players which will add to the energy in our group and I know he will put his all into this role with us.
"With the level of experience and professionalism Sean has, he will hit the ground running with the knowledge he has of our region, players and the game in Wales."
Sean Holley who joins the Scarlets officially next week said: "I'm looking forward to working with Simon and his team - they are a hard-working and committed coaching team with clear goals, real ambition and a strong set of values.
"It's a close-knit group at Parc Y Scarlets. As a team, the Scarlets are destined to achieve success with the number of exciting players they have with real talent and passion for the shirt they wear. They have worked hard to put themselves in a strong position in the RaboDirect PRO12 and they need to cement that in coming weeks.
"It will be a great environment to experience with an excellent stadium and rugby facilities and I hope to add real value to the drive and ambition at Parc y Scarlets for the time I am here."
Sean Holley will be working as backs and skills coach in the interim period while Mark Jones is away.