The multiverse is a setting within the Marvel Cinematic Universe (MCU) media franchise. Based on the setting of the same name from the Marvel Comics, it is a collection of infinitely many alternate realities and dimensions. First explored in the film Doctor Strange (2016), it is revisited in the film Avengers: Endgame (2019) before playing a key role in Phases Four, Five, and Six of the MCU, which constitute "The Multiverse Saga". The MCU's multiverse centers on a universe sometimes referred to as the "Sacred Timeline", which also describes a bundle of closely aligned, infinite timelines that do not lead to the emergence of a Kang variant. Initially, branched timelines were generally "pruned" by the Time Variance Authority (TVA), until Sylvie kills TVA's leader, "He Who Remains", allowing the multiverse to be freed and new branched timelines to fully form. Following this, the TVA under new management instead nurtures and observes these new universes. Occasionally, some universes are considered separate from the "Sacred Timeline" but still monitored by the TVA.

Many alternate versions of existing MCU characters have been introduced via the multiverse, most notably those of Loki, Kang the Conqueror, Spider-Man, Doctor Strange, Deadpool, and Wolverine. The main reality depicted in the MCU is designated Earth-616 beginning with the film Doctor Strange in the Multiverse of Madness (2022), despite its original designation as Earth-199999 by Marvel Comics and outside media. The multiverse has received a mixed response from critics, with praise for its visuals and nostalgic appeal but criticism of its excessive reliance on fan service. The incorporation of characters from non-MCU Marvel films were featured in the films Spider-Man: No Way Home (2021) and Deadpool & Wolverine (2024) which has also generated speculation and discussion among viewers and commentators.

The multiverse was first introduced to the Marvel Comics during the 1960s and 1970s. In Strange Tales #103 (1962), the character Johnny Storm of the Fantastic Four is teleported to an alternate reality for the first time in Marvel history, with the character sent to the Fifth Dimension. The concept of the multiverse was then fully explored in What If...? #1 (1977) and Marvel Two-in-One #50 (1979), with the term "multiverse" first used in the What If...? series. The main reality featured in the comics, Earth-616, was first named in The Daredevils #7 (1983) by Captain Britain creator David Thorpe to differentiate the character from his alternate versions.

In 2008, the film Iron Man was released, kickstarting the Marvel Cinematic Universe (MCU) media franchise. The setting of the franchise was subsequently designated Earth-199999 by Marvel Comics in the hardcover version of the Official Handbook of the Marvel Universe A-Z, Vol. 5 (2008). The characters Loki, Gwenpool, and Doctor Strange from the comics have been shown to be aware of the MCU's existence. The multiverse is introduced to the MCU in the film Doctor Strange (2016), with director Scott Derrickson noting that the character in the comics had previously "broke[n] open the Marvel comic book universe into the Marvel multiverse". At the time, producer and Marvel Studios president Kevin Feige stated that there were no plans to explore parallel universes similar to the ones featured in the comics, with the film instead exploring various "alien dimensions". It is later confirmed that the infinite dimensions of the multiverse explored in Doctor Strange (2016) are all contained within individual timelines.

The MCU multiverse is revisited in the film Avengers: Endgame (2019), in which the Avengers journey to four alternate timelines as part of a "Time Heist". The escape of an alternate version of Loki from an alternate 2012 New York sets up the first season of the Disney+ series Loki (2021). The multiverse plays a central role in Phase Four of the MCU, most notably in the first season of Loki, the first season of the Disney+ series What If...? (2021), the film Spider-Man: No Way Home (2021), and the film Doctor Strange in the Multiverse of Madness (2022). Phase Four, Phase Five, and Phase Six of the MCU will comprise "The Multiverse Saga".

The showrunners of Loki collaborated with the crews behind the Disney+ series WandaVision (2021), What If...?, and the film Ant-Man and the Wasp: Quantumania (2023), as all three projects are connected to the multiverse in some way. Together with WandaVision co-executive producer Mary Livanos and What If...? executive producer Brad Winderbaum, Loki executive producers Kevin Wright and Stephen Broussard developed a "rule book" regarding the MCU's multiverse and alternate timelines. Feige also held a meeting with Marvel Studios executives to discuss the rules of the multiverse and how they would present it to audiences.

With the release of The Marvel Cinematic Universe: An Official Timeline in October 2023, Feige wrote in its foreword that Marvel Studios only considered, at that time, projects developed by them in their first four phases as part of their "Sacred Timeline", but acknowledged the history of other Marvel films and television series that would exist in the larger multiverse given they were "canonical to Marvel". Additionally, he noted as Marvel Studios progressed in the Multiverse Saga, other outside timelines had the potential to "crash or converge" with the Sacred Timeline.

In Doctor Strange, the term "multiverse" is used by the Masters of the Mystic Arts to describe the multitude of dimensions within the MCU. The character Ancient One brings Dr. Stephen Strange on a journey across the multiverse, passing by different universes and pocket realities, including the Quantum Realm introduced in the film Ant-Man (2015), the Mandelibus Dimension, the Actiniaria Dimension, the Flowering Incense Dimension, and the Grass Jelly Dimension. Internally referred to as the "Magical Mystery Tour" at Marvel Studios, the scene was originally seven minutes long. In an unrealized scene in the film Avengers: Infinity War (2018), Strange sends Thanos through a series of alternate universes, emulating the Magical Mystery Tour. The film most notably explores the Astral Dimension, the Dark Dimension, and the Mirror Dimension.