Garth played one of the most popular characters on the original, long-running "90210" show.
Although the new version of the teen soap is expected to have a different, younger cast, rumors have been rampant that some of the original show's stars, most of whom are now in their 30s, might return in supporting roles.
The Hollywood Reporter said Garth left "Girl" after a table read Monday. The trade paper said the decision for her to depart was a mutual one made by her and the show's producers.
Canadian actress Kristin Booth has signed on to replace her.
The Hollywood Reporter said Rob Thomas, who wrote the pilot for the "90210" spinoff, has been quoted in the media as saying he would like to see Garth reprise her role of Kelly Taylor on the new show.