The experienced Welsh coach will work alongside head coach Simon Easterby, while current Scarlets attack coach Mark Jones joins the Wales set-up for the RBS Six Nations campaign.
The 42-year-old, who is the longest serving Welsh regional coach, left his Ospreys role almost a year ago after joining in 2003, having led them to a string of notable victories.
Looking forward to joining the Scarlets next week, Holley 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."
Easterby is excited for Holley's arrival and said the Welshman's mass experience would bring a great deal to the Parc y Scarlets.
"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," he said.
"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."
Easterby added: "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.
Source : http://www1.skysports.com/rugby-union/news/12332/8406313/Sean-Holley-joins-the-Scarlets-coaching-set-up-in-the-absence-of-Mark-Jones
