The extension will take Galthie up to the World Cup after next year's tournament in France.
"It's done. It's going to be extended until 2027," FFR vice-president Serge Simon said.
"When you are lucky enough to have something that is so obviously right, you have to keep hold of it."
France won their first Six Nations title since 2010 by defeating England 25-13 in a superb all-round display at the Stade de France on Saturday.
The victory also gave Les Bleus their first Grand Slam in 12 years and underlined their status as one of the favourites to win the 2023 World Cup on home soil.
Galthie's previous contract would only have taken him until the 2023 World Cup.
Galthie, a former France captain, took over after the 2019 World Cup in Japan.
The 53-year-old won 64 caps as a scrum-half with Les Bleus, and lifted the Six Nations trophy three times as a player in 1997, 1998 and 2002.
