If the deal comes through, DeGeneres would voice Dory, a naive but good-hearted and optimistic regal tang with short-term memory loss, in the sequel to be released in 2016, The Hollywood Reporter said.
DeGeneres has several times said on her syndicated talk show she hoped a "Nemo 2" would be made.
Andrew Stanton, who wrote the original 2003 film with DeGeneres in mind and co-directed it, is to direct the sequel. Stanton also directed "WALL-E" in 2008 and most recently the science fiction action film "John Carter."
The original "Nemo," which won the Academy Award for Best Animated Feature, also stars Albert Brooks as overly protective clownfish Marlin, who searches with Dory for his abducted son Nemo, played by Alexander Gould.
The newspaper did not say if Brooks or Gould were also in talks to reprise their roles. The Internet Movie Database lists Brooks as the "rumored" voice of Marlin. It lists 12-year-old Preston Bailey as the rumored voice of Nemo.
It also lists Jane Kaczmarek, who played the character of Lois on the TV sitcom "Malcolm in the Middle," as the rumored Dory voice.
The original "Nemo" is the highest-grossing Pixar Animation Studios film, earning $868 million to date worldwide. It is getting a 3-D update and is to be re-released in theaters next month.