The news may come as a surprise since Weaver's character, Grace Augustine, was killed in James Cameron's 2009 film.
However, Cameron says Weaver will return as a "more challenging character" in the upcoming sequels.
"Sigourney and I have a long creative history, dating back to 1985 when we made 'Aliens,'" said Cameron in a studio statement. "We're good friends who've always worked well together, so it just feels right that she's coming back for the Avatar sequels."
Weaver will reteam with Sam Worthington, Zoe Saldana, and Stephen Lang.
"Avatar" became the highest-grossing film ever making $2.8 billion at theaters.
The next three films are due out December 2016, 2017, and 2018.