Colin Firth, who plays Bridget's husband Mark Darcy, Patrick Dempsey, whose role is not leaked yet, will also be seen.
Hugh Grant, who played Bridget's other love interest, will not return for the third film, reported E!online.
"I'm really glad to be doing it," Zellweger said.
The first official image from "Bridget Jones's Baby" was released around two weeks ago.
The movie is adapted from Helen Fielding's columns for a UK newspaper.