Galthie, 50, won 64 caps as a scrum-half and will work as an assistant to Brunel in the build-up to the tournament which starts on September 20, before taking over permanently on a contract until 2023 — when France will host the competition.
Galthie has had spells at the helm of French clubs Montpellier, Toulon and Stade Francais where he won the Top 14 title.
Brunel has experienced a difficult tenure since replacing the sacked Guy Noves in 2017, winning only five games in 16 fixtures.
France will face England, Argentina, the United States and Tonga in the World Cup pool stage in Japan after warm-up matches against Scotland and Italy this summer.
Ireland head coach Joe Schmidt was the FFR president Bernard Laporte’s first choice for the position but French rugby clubs showed their opposition to a foreigner being appointed in a referendum last week.
Laporte had opened the debate in the wake of Les Bleus finishing a disappointing fourth in this season’s Six Nations.
The result of the clubs’ votes capped a traumatic week for the FFR, which was ordered on April 8 by an industrial tribunal to pay one million euros in compensation to the sacked Noves.
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