Spelljammer is a campaign setting originally published for the Advanced Dungeons & Dragons (2nd edition) role-playing game, which features a fantastic (as opposed to scientific) outer space environment.

Spelljammer introduced into the AD&D universe a comprehensive system of fantasy astrophysics, including the Ptolemaic concept of crystal spheres.

With their own fields of gravity and atmosphere, the ships have open decks and tend not to resemble the spaceships of science fiction, but instead look more like galleons, animals, birds, fish or even more wildly fantastic shapes.

The Spelljammer setting is designed to allow the usual sword and sorcery adventures of Dungeons & Dragons to take place within the framework of outer space tropes.

Flying ships travel through the vast expanses of interplanetary space, visiting moons and planets and other stellar objects.

However, unlike Planescape, it keeps all of the action on the Prime Material Plane and uses the crystal spheres, and the "phlogiston" between them, to form natural barriers between otherwise incompatible settings.

Though the cosmology is derived largely from the Ptolemaic system of astronomy, many of the ideas owe much to the works of Jules Verne and his contemporaries, and to related games and fiction with a steampunk or planetary romance flavor.

Shannon Appelcline, in the book Designers & Dragons (2011), highlighted that in 1989 Spelljammer was the first of a host of new campaign settings published by TSR.

It was created by Jeff Grubb and "introduced a universe of magical starships traversing the 'crystal spheres' that contained all the earthbound AD&D campaign worlds.

It suggested a method to connect together all of TSR's settings and at the same time introduced fun new Jules Verne-esque technology that had never before been seen in the game.

[1]: 26  Appelcline commented that Spelljammer "offered a way to connect every single D&D fantasy world, was thus one of the first true crossovers" in role-playing games.

[2] Several of TSR's other campaign worlds had their own sections in the Spelljammer Boxed Set - Realmspace for the Forgotten Realms, Krynnspace for Dragonlance, and Greyspace for Greyhawk.

[citation needed] The product line would be expanded with a number of boxed sets and accessories such as Lost Ships (1990),[3] Realmspace (1991)[4] and The Astromundi Cluster (1993).

[14] The book included a chapter with a sample map of a crashed Spelljamming vessel, cultural habits of the neogi, and the monster's stat blocks.

[17][12][18] Then in October 2021, Wizards released the PDF Travelers of the Multiverse which is part of the "Unearthed Arcana" public playtest series.

[20] A prequel adventure module, titled Spelljammer Academy, was released for free on the Wizards of the Coast website and on D&D Beyond in July of the same year.

As a living thing (although it does not consume any matter, it does absorb heat and light through its ventral (or under) side and uses them to produce air and food for its inhabitants), the Spelljammer has a complex life cycle and means of procreation.

Alien races inhabiting the Spelljammer universe included humans, dwarves, xenophobic beholders, rapacious neogi, militant giff (humanoid hippopotami), centaurlike dracons, hubristic elf armadas, spacefaring orcs called "scro", mysterious arcane, the Thri-kreen insectoids, and bumbling tinker gnomes.

Spelljammer has acted as the official campaign setting for multiple Dungeons & Dragons roleplaying adventure modules, sourcebooks and accessories.

It will also feature guest stars such as Brennan Lee Mulligan, Aabria Iyengar, Ginny Di, Anna Prosser, Deejay Knight, Emme Montgomery, Travis McElroy, SungWon Cho, and Jim Zub.

[30] Alexander Sowa, for CBR in October 2021, commented that Spelljammer should be one of the classic settings Wizards of the Coast brings back for the 5th Edition.

Wizards tossed them a bone with the Dream of The Blue Veil spell added in Tasha's Cauldron of Everything, but it's not a replacement for the niche Spelljammer previously filled.

It's not just a way to travel between different campaign settings; it's a simultaneous fulfillment of sci-fi and fantasy dreams of exploration, venturing deep into unknown depths and contending with the strange and otherworldly".

[33] In his 2023 book Monsters, Aliens, and Holes in the Ground, RPG historian Stu Horvath called the game "gloriously bewildering."