Spelling, 40, and Garth, 41, had played Donna Martin and Kelly Taylor, respectively, in "Beverly Hills, 90210", reports people.com.
"Mystery Girls" revolves around two woman who start solving crime mysteries.
While, Spelling plays Holly, a former actress desperate to regain her fame, Garth will be seen as Charlie, Holly's former co-star.
Both Spelling and Garth will also serve as executive producers.
"Mystery Girls" is scheduled to start shooting in November.