What was the first 3D animated film?

The first 3D animated film is generally considered to be "The Adventures of André and Wally B."

This short film was created in 1984 by John Lasseter at Pixar, while it was still known as Lucasfilm's computer division.

While it wasn't released theatrically, it was a significant step in the development of 3D animation and paved the way for future Pixar films.