Rain World is a 2017 survival-platform video game developed by independent studio Videocult and published by Adult Swim Games and Akupara Games. It was released for PlayStation 4 and Windows in March 2017 and Nintendo Switch in December 2018. The player guides a "slugcat", an agile animal resembling a cat-slug hybrid, as it fights for survival in a derelict and hostile environment. The slugcat traverses the remnants of an industrialized ancient civilization in search of its lost family. It uses debris as weapons to ward off lethal predators, scavenges for food, and attempts to reach safe hibernation rooms before deadly torrential rain arrives. Rain World can be played alone or with multiple players; there is also a sandbox mode.

Development began in 2011 and included a successful Kickstarter campaign in 2014. The two-man team intended to simulate a unique, realistic ecosystem, in which creatures act independently from the player and perpetually roam the environment. Rain World uses real-time procedural animation and conveys much of its narrative through environmental storytelling, adopting an adaptive low-fi and electronic soundtrack. The player is given little explicit guidance so that they would feel like a rat that lives on subway tracks, learning to survive in an environment without comprehending its higher-level function.

Rain World received mixed reviews from critics, who praised its art design and fluid animation but criticized its difficulty, checkpoint system, and controls; some of these concerns were addressed with later updates. The game was nominated for awards and garnered a cult following and a large modding community. A downloadable content pack titled Downpour, adapted from a popular community mod, was released for Windows in January 2023 and ported to consoles in July 2023. It added five slugcat characters with unique abilities and was positively received by critics. A second content pack, The Watcher, was released for Windows in March 2025 and ported to consoles in September.

Rain World is a two-dimensional (2D) survival-platform video game taking place in a post-apocalyptic, industrial environment. The game follows the player-controlled "slugcat", an animal akin to a cat and a slug. The slugcat freely explores screens shown individually by squeezing through pipes or walking; some screens spawn creatures that wander the region, which the slugcat may defend against with spears and debris. It must jump, swim, and climb poles to avoid enemies while foraging for food which must be consumed to hibernate in scarce, designated safe rooms. Hibernation spots serve as checkpoints where it returns to after death; if the slugcat does not reach a shelter before the end of the cycle, rain will kill it in an ensuing flood.

Upon death, the slugcat loses one "karma" level, which is gained upon hibernating. The slugcat can prevent losing karma by eating a yellow "karma flower". The flower appears in set locations and is replanted wherever the slugcat dies after eating it. It must meet a specific karma level to pass through gates set at the borders of the game's 12 regions. Rain World offers little to guide the player apart from holograms by a worm-like creature that monitors the slugcat. The holograms point towards food and story-related events, though this assistance becomes rarer over time. The player may view an in-game map to review their exploration progress.

Predators range from camouflaged carnivorous plants to large gas-producing vultures and colored Komodo dragon-like lizards; some creatures are friendly but easily provoked, like the "scavengers", which are agile like the slugcat and equipped with explosive spears. Many enemies can kill the slugcat in one attack. Some have variations that possess unique characteristics. All creatures possess dynamic behavior and perpetually wander independently from the slugcat, occasionally battling and hunting each other; without a set path for predators to traverse, the slugcat's exploration is unpredictably hindered. The slugcat may carry two items with its hands. It uses its right hand first when throwing and may swap the items' places. Some foods grant status effects when consumed, like slowing down time.

Along with the default slugcat, the player may choose to play as the Monk and Hunter slugcats. As the Monk, creatures are less aggressive and the slugcat needs less food to hibernate. The Hunter, a carnivore with a bigger appetite, must fight and feed on larger creatures while also being capable of carrying extra spears. Other game modes include a multiplayer arena mode, where up to four players battle each other, and a sandbox mode, where the player freely spawns and interacts with objects and creatures.

Rain World has two downloadable content packs (DLCs), the first being Downpour. Downpour triples the game's world size and adds five slugcats and ten regions, accessed separately from the original game's content. Each slugcat features a different set of abilities. The Artificer can jump twice midair and create explosives. The Spearmaster produces an infinite amount of spears, but piercing other creatures is the only way it receives food. The Rivulet, a semiaquatic slugcat, has increased agility, but must deal with more frequent rain. The Gourmand can craft usable gear by merging items, though it requires a tremendous amount of food to stay alive. The Saint has a tongue that can grapple onto objects, granting it high mobility, but cannot throw spears and is prone to freezing to death.