Which Toy Story film was Pixar's first sequel?

Toy Story 2 was Pixar's first sequel.

The film was released in 1999 and continued the story of Woody, Buzz Lightyear and Andy's other toys. It began as a direct-to-video project but was later upgraded to a theatrical feature after Pixar and Disney recognized its potential.

Toy Story 2 expands the world of the first film by introducing Jessie, Bullseye, Stinky Pete and Al McWhiggin. Its story also explores Woody's value as a collectible and his choice between museum preservation and life with Andy.

Toy Story 2 is not Pixar's first feature film—that distinction belongs to Toy Story. It was, however, the studio's first follow-up feature, preceding later sequels such as Toy Story 3 and Finding Dory.