Lee Unkrich revealed on Twitter that he has started work on the movie and will be charting his progress through his Tumblr page.
"Today is day one on my next project," he wrote. "Every day I'll post a new photo chronicling the journey."
The first two photos published by Unkrich are of a keyboard - with the caption 'The starting line' - and a plate with a sandwich and bowl of vegetables on it.
Unkrich is yet to announce the name or any details of the project, but it is not thought to be either of the two movies that Pixar teased in August.
Toy Story 3 was nominated for 'Best Adapted Screenplay' at the 2011 Academy Awards. It was also nominated for three BAFTAs, taking home the prize for 'Best Animated Film'.
Watch the trailer for Toy Story 3 below: