The 59-year-old actor will write and direct "Rocky Balboa," which will begin shooting in Philadelphia and Las Vegas next year.
Stallone told the Daily Variety trade magazine the movie will focus on an aging, widowed Rocky who is reluctant to get back in the ring but ends up doing it "just to compete, not to win."
"I am drawing on a lot of my feelings that are in sync with many people's feelings about facing the last chapter of their lives and how they want it to be written," Stallone said.
Dan Taylor, president of Metro-Goldwyn-Mayer, which is co-producing the film, said the movie focuses less on boxing and more on character.
"It's about self-examination, an opportunity to see what kind of person he is and act on it," Taylor said.
It wasn't immediately clear whether Talia Shire, who appeared in five previous "Rocky" films as the boxer's love interest, Adrian, will return in the sixth installment, said Stuart Herriot, a spokesman with Revolution Studios, which is co-producing the film.
"In the script, she did pass away, but it's unsure whether she might appear in a flashback or in older footage," Herriot said. "It's too early in the production to tell."
But, Herriot said, Burt Young is expected to return as Adrian's brother, Paulie.
Stallone wrote all the "Rocky" films and directed the second, third and fourth entries.
The first movie, released in 1976, won three Oscars, including best picture and best director for John G. Avildsen.