Timu, 25, has Samoan roots, was born in New Zealand and grew up in Brisbane.
He started his professional career in rugby league, taking a two-year break during his time with the Brisbane Broncos to serve as a Mormon missionary.
He moved to the Queensland Reds in Super Rugby in 2018 and has played two tests for the Wallabies.
He joins a Montpellier squad that already contains French international back-row forwards Louis Picamoles, Fulgence Ouedraogo, Kelian Galletier and Yacouba Camara.
Montpellier were knocked out in the first round of the Top 14 playoffs in the 2018-2019 season.