The former Ospreys head coach will work alongside Simon Easterby and his coaching team.
It follows the announcement earlier this week that Scarlets attack coach Mark Jones will be part of the Wales coaching unit for the Six Nations, which starts on February 2.
Holley, 42, will work as backs and skills coach while Jones is away from Scarlets duty.
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 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 we need to make significant progress in the RaboDirect PRO12.
"We are fortunate, given the time-frame and being midway through the season, to have secured the services of someone of Sean's capability and background."
- Sports & Recreation