Fat Bear Week is an annual event held in October by Katmai National Park and Preserve in Alaska, celebrating the impressive seasonal preparations made by brown bears inhabiting Katmai as they ready themselves for their winter hibernation. The competition is jointly organized by the National Park Service and Explore.org, and human spectators from around the world are invited to participate by casting online votes to determine the bear that has most effectively accumulated fat reserves. The event was founded as Fat Bear Tuesday by former park ranger Mike Fitz in 2014, and the tournament went digital and was extended to a week in 2015.

During the winter, bears enter their dens where they will not eat or drink until they emerge in spring. During this time, they may lose up to one-third of their body weight as they rely solely on their fat reserves. Survival depends on eating a years worth of food in six months. Fat Bear Week is a way to celebrate the resilience, adaptability, and strength of Katmais brown bears.

Fat Bear Week takes place over the course of seven days and concludes on Fat Bear Tuesday, where people vote for the biggest, best bear. The bears are chosen based on things like having before-and-after pictures of the skinny spring bear and the fat fall bear, and if the animal has wandered onto the livestream at Brooks Falls that year. In addition to voting during this week-long tournament, there are also a series of online chats on explore.org featuring Mike Fitz, explore.orgs resident naturalist, and rangers from Katmai. These chats focus on both the individual bears and Katmais healthy ecosystem.

Fat Bear Week 2023 occurred from October 4 – October 10, and the matchups and results can be found on the National Park Service website. The money raised from the buy-in was donated to a charity chosen by the bracket winner, such as the Katmai Conservancy.