Mjölnir (commonly written without diacritics as Mjolnir) and Stormbreaker, in the Marvel Cinematic Universe (MCU), are sentient enchanted weapons of choice used by Thor. Both are melee weapons and were created out of Uru metal forged with the heat of a dying star in the Dwarven kingdom of Nidavellir, with the assistance of the dwarf king and master weapon-maker Eitri. Mjolnir is a hammer, and was enchanted by Thor's father, Odin, so that only those the hammer deemed "worthy" are capable of wielding or even lifting it. Stormbreaker is an axe, and although it does not have such a worthiness enchantment, its power is such that a mere mortal attempting to wield it would be driven mad.

Mjolnir first appeared in the post-credits scene in Iron Man 2 (2010), and has appeared thereafter in every film featuring Thor except for one, those being Thor (2011), The Avengers (2012), Thor: The Dark World (2013), Avengers: Age of Ultron (2015), Doctor Strange (2016), Thor: Ragnarok (2017), Avengers: Infinity War (2018), Avengers: Endgame (2019), and Thor: Love and Thunder (2022). Stormbreaker first appears in Avengers: Infinity War (2018)—also the only MCU film in which Thor does not wield Mjolnir at all—and appears again in Avengers: Endgame, and Thor: Love and Thunder. Conversely, the post-credit scene in Iron Man 2 and a scene from the second episode of the Disney+ TV series What If...? are the only MCU media in which Mjolnir appears while Thor does not.

Mjolnir debuted in print in the Marvel Comics title Journey into Mystery #83 (Aug. 1962), being the means by which physician Donald Blake transformed into thunder god Thor Odinson (by striking it on the ground). The first use of the hammer's name was in the "Tales of Asgard" feature in Thor #135 (Dec. 1966) in a story by Stan Lee and Jack Kirby. The weapon's origin is eventually revealed in Thor Annual #11 (1983), with another version presented in Thor vol. 2, #80 (Aug. 2004).

Mjolnir's origin in the Marvel Comics continuity mirrors the original Norse legend. The hammer is created when Odin's adopted son Loki cuts off the hair of the goddess Sif as part of a cruel jest, and, when threatened with violence by Thor, promises to fetch replacement hair from the dwarf smiths. Loki commissions the hair from the Sons of Ivaldi, and the obliging dwarves create the hair and other gifts for the gods. Loki is convinced that no one can match their workmanship, and challenges a dwarf named Eitri to make finer treasures. After creating some other magical items, Eitri begins work on a hammer. Loki panics, fearing that he will lose the wager, and transforms himself into a moth to bother Eitri (in the original myth, Loki becomes a gadfly and stings Eitri's assistant). As a result, the hammer's handle is shorter in length than Eitri intended, meaning that it could only be wielded one-handed. Despite the error, the Norse gods consider Eitri to have forged the greater treasures, and Loki loses the bet.^[1]

In a 2002 documentary with Kevin Smith, Lee says his brother and co-creator Larry Lieber originally referred to Mjolnir as the "Uru Hammer".^[2] Writer Roy Thomas eventually changed the name of the hammer to the mythologically correct name of "Mjolnir" but maintained the Larry Lieber concept of it being composed of fictional metal "uru".^[3]

Visual Development Supervisor Charlie Wen was tasked with designing Thor's appearance for the films, and focused on mixing elements from the comic books with Norse mythology, "trying to maintain the Norse side of things" as much as possible.^[4] The first design element that Wen attempted was Mjolnir, for which Wen created a number of possible alternatives, incorporating designs including "the traditional Thor hammer with the short handle as well as the Ultimates versions", from which the one director Kenneth Branagh chose "was the most traditional one".^[4] In a 2018 interview with Jimmy Kimmel, Thor actor Chris Hemsworth revealed that he has kept a number of the prop hammers created for the film.^[5]

The concept artist tasked with designing the MCU version of Stormbreaker, Ryan Meinerding, decided to deviate substantially from the design of Stormbreaker used in the comics, feeling that "the original Stormbreaker looked a bit too much like the original Mjolnir".^[6] Consequently, Stormbreaker "more closely resembles the Mjolnir from the Ultimate Marvel run than the original Stormbreaker hammer bestowed upon Thor's rival turned ally Beta Ray Bill".^[6] Meinerding intentionally made Stormbreaker comparatively oversized and overpowered, to convey that the wielder of the weapon must also be someone incredibly powerful.^[6]

Both weapons can be summoned by Thor, and will return to his hand after being thrown. Both weapons also enable Thor to fly, and to channel his power to summon lightning. Stormbreaker has the additional power to summon the Bifröst, allowing Thor to teleport anywhere in the Nine Realms. Screen Rant has identified Stormbreaker as the more powerful of the two, based on this ability, as well as it being a much larger and edged weapon.^[7] Both weapons also appear to have a degree of sentience, with Stormbreaker apparently being envious of Thor's affection for Mjolnir,^[8] a characteristic director Taika Waititi attributed to the handle being made from the arm of the adolescent Groot, and carrying over some of that character's moodiness at that age.^[9]

Mjolnir also appears in a June 2014 poster advertising the upcoming film, Ant-Man, with the protagonist in his miniaturized form standing on the hammer, though it does not appear in the film itself.^[10]

A broken, alternate version of Mjolnir owned by Throg appears in the fifth episode of Loki in the Void.^[15] In the same episode, the Loki variant described as Boastful Loki "wields a hammer that looks an awful lot" like Mjolnir.^[16]

Another alternate version of Mjolnir appears in the second episode of What If...? as part of the Collector's collection on Knowhere.^[17] In the third episode, the hammer appears in the desert as it had in the film, Thor, but Thor is fatally wounded by an arrow from Clint Barton while trying to retrieve it.^[18] Still another version appears in the seventh episode, which Thor uses to fight against Captain Marvel. For unexplained reasons, Thor remains the only one able to lift the hammer in this universe despite Odin never being shown enchanting it with the "worthy" spell.^[19] When Thor's mother, Frigga, arrives to check on him, and he pretends to have been studying Earth culture the entire time, his ruse is revealed when he summons Mjolnir, which is covered with paint, party beads, and a pair of underwear. Thor later uses the hammer to battle a multiverse-threatening version of Ultron.

The MCU TV series Hawkeye features a fictional in-universe performance of Rogers: The Musical, a Broadway musical featuring an actor playing Thor wielding a prop Mjolnir.^[20][21]

In the What If...? series season 2, episode 2 shows Thor fighting with Mjolnir alongside a team formed by Howard Stark and Peggy Carter in 1988 against Peter Quill and his father, Ego. In "What If... Hela Found the Ten Rings?", Mjolnir was still wielded by Hela until (in this timeline, where Odin banished Hela to Earth instead of Hel) Odin destroyed the hammer the same way Hela did in Thor: Ragnarok.

In the final episode of What If...? season 2, it is revealed that Strange Supreme has been collecting heroes and villains from various universes to feed to a magical forge, including multiple of versions of Thor. As Strange attempts to send one version of Thor into the forge, that version throws Mjolnir to Peggy Carter and Kahhori, the latter of whom wields it, along with the Ten Rings bequeathed to her by an alternate version Xu Wenwu, in the fight against Strange.^[22]