The 23-year-old rejoins the world champions, where he performed the same role in 2008, from Red Bull-backed Toro Rosso. Buemi raced there for three seasons before being released at the end of last year.
"It's good to remain with Red Bull for another year and have this opportunity with the World Championship-winning team," the Swiss said.
"I would prefer to be driving at the races of course, but working with Red Bull Racing on the development of their car and providing them with feedback throughout the season is the next best thing."
Red Bull team principal Christian Horner added: "It was entirely logical for Sebastien to continue with Red Bull having been a member of the Junior Team since 2004. He already knows the team well from his time with us in 2008 and has gained significant race experience over the past three years.
"He will complete important test and simulation work for us throughout the year and will be in attendance at each grand prix."