The third and final season of the American animated anthology series What If...?, based on the Marvel Comics series of the same name, explores alternate timelines in the multiverse that show what would happen if major moments from the films of the Marvel Cinematic Universe (MCU) occurred differently. The season is produced by Marvel Studios Animation, with Matthew Chauncey serving as head writer and Bryan Andrews and Stephan Franck directing. Animation for the season is provided by Flying Bark Productions and Stellar Creative Lab, with Scott Wright serving as head of animation.

Jeffrey Wright stars as the Watcher, who narrates the series, alongside numerous MCU film actors reprising their roles. Development began by July 2022, with Chauncey replacing previous head writer A. C. Bradley for the season by December 2023. Andrews and Franck returned from previous seasons to direct.

The third season premiered on the streaming service Disney+ on December 22, 2024, with its remaining episodes released daily until December 29. It is part of Phase Five of the MCU.

The series is narrated by Jeffrey Wright as the Watcher, a member of the alien Watcher race who observes the multiverse. Each episode features different versions of characters from the MCU films, with many actors reprising their roles for the series.

Work had begun on a third season of What If...? by July 2022. At that time, head writer A. C. Bradley revealed that the second season of the series was her final project at Marvel Studios, with series story editor Matthew Chauncey taking over as head writer for the season. Series director Bryan Andrews returned for the third season along with Stephan Franck. The season consists of eight episodes. Franck was able to assist Andrews in the season while the latter was simultaneously working on Marvel Zombies (2025); Andrews was still able to supervise much of the season and ultimately directed three of the season's episodes and co-directed with Franck for one episode, with Franck directing the other four episodes of the season. A Red Guardian–centered episode, written by Bradley in 2020, is included in the season after originally being intended for the second season. Executive producers for the season include Brad Winderbaum, Kevin Feige, Louis D'Esposito, Dana Vasquez-Eberhardt, and Andrews. In August 2024, it was announced that the third season would be the series' last. That November, Winderbaum explained that the series was ending due to "bigger [MCU] reasons" that were being determined in real time as different projects were released within the Multiverse Saga. He said it was the right time for the series to conclude "from a story perspective", but did not rule out reviving it in the future.

Bradley and Ryan Little served as writers on the season alongside Chauncey. Winderbaum described the season as being the culmination of a trilogy, completing an "emotional experience" with the Watcher whose humanity is explored more than in previous seasons. He added that the season would have the first hints at "crossover potential" between the series and Marvel Studios' other animated projects. Some of the unused episode concepts that were created for the first and second season were used for the third. Andrews said the third season's episode concepts were "wackier" than those used for the first two seasons, and further explore different genres.

Episodes include: an anime-inspired episode featuring the Avengers piloting mechs to fight a horde of Mega-Hulk monsters, titled "Go-Avengers: Heroes of the Gamma War", with Andrews excited to go "full, hardcore, mega genre" with the episode; one with a "1930s screwball" Bollywood musical number featuring Agatha Harkness and Kingo; one that sees Red Guardian stopping the Winter Soldier from assassinating Howard and Maria Stark and the two becoming allies; one that revisits Howard the Duck and Darcy Lewis from the first season episode "What If... Thor Were an Only Child?", who have since gotten married and must protect their child from various threats; one where an alternate take on the events of Eternals (2021) leads to the destruction of Earth and the rise of an authoritarian regime led by Quentin Beck; and a Western-themed episode, a genre which Andrews had wanted to explore since the first season.

Ideas that were considered for the season but not used include trying to involve the band Kiss in six different episodes, one of which would have been a galactic Battle of the Bands concept with Kiss serving as the judges; a samurai episode that Andrews described as "hardcore and just brutal and awesome" with elements of Romeo and Juliet; a Dungeons & Dragons-styled episode; an episode centered on Sam Wilson and Yelena Belova where the latter would be the Winter Soldier; and a kung fu-themed episode featuring Iron Fist and Xu Shang-Chi. Andrews also wanted to include the characters Beta Ray Bill and Ghost Rider, but Marvel informed him that they were off-limits because they may appear in future projects and wanted to prevent the writers from telling similar stories. Furthermore, despite technically having access to X-Men characters due to X-Men '97 (2024–present) being released prior to the season, Andrews and his team were unaware of whether they had permission to do so during development. In retrospect, Andrews expressed regret towards not using Hugh Jackman's Logan / Wolverine.