Scarlets chief executive Stuart Gallacher said: "We're delighted to welcome Ryan. He has a wealth of experience in conditioning.
"As one of Wales' top fitness coaches, Ryan will complement the work of our new head conditioning coach Brad Harrington. We're confident that, together, they'll provide the experience and expertise required at professional rugby's elite level."
Campbell added: "I'm thrilled to have the chance to work with the Scarlets. They have a proven pedigree in European rugby and are extremely ambitious.
"The new stadium is a statement of intent and I'm looking forward to being involved in a world class environment."
Harrington is expected to arrive at Stradey Park in the coming week, as is new defence coach John Muggleton. Head coach Nigel Davies' team also includes forwards coach Paul Moriarty and team manager Garan Evans.
The Scarlets expect to move into their new £23m Parc y Scarlets in November. The complex features a stadium with a capacity of around 14,500 and top-grade training facilities.