Players assume control of a "slugcat", an elongated felid-like rodent, and are tasked with survival in a derelict and hostile world.

The slugcat uses debris as weapons to escape predators, scavenges for food, and tries to reach safe hibernation rooms before the deadly torrential rain arrives.

[3][4] The player is given little explicit guidance and is free to explore the world in any direction[4] by entering pipes and crawling through passages that span across over 1,600 static screens that each spawn their creatures in set locations.

[5][6][7] The slugcat can jump, swim, and climb poles to avoid enemies while foraging for sparse food, which is used to hibernate in scarce, designated safe rooms called shelters.

If the player does not reach a shelter before the end of the day, rain will come, crushing the slugcat or causing them to drown in one of the many now-flooded rooms.

Creatures spawn from dens in set locations and can move freely throughout the region, meaning the player is sometimes faced with problems that they can't avoid.

[9] Players are expected to mainly evade enemies,[10] but do need to experiment with spears to climb walls and knock fruit off of vines.

[11][12] The game offers very little to guide the player aside from the overseer, who gives some hints about where to find nearby shelters and lore-related events.

Gourmand has access to a crafting system and can damage creatures by falling or sliding into them, but excessive physical activities cause them to enter a weak-winded state where they must stop to catch their breath.

Downpour's release was accompanied by the free Rain World Remix upgrade, which added accessibility options, ways to customize game difficulty, and better modding support.

She explains that the ancient civilization that once inhabited the world sought to escape the eternal cycle and succeeded, collectively ascending from existence and abandoning the iterators.

Eventually, Five Pebbles decided to create an organism to overwrite the "self-destruction taboo", a law written into all of the cells of all iterators to stop them from escaping the cycle of death and rebirth themselves.

Looks to the Moon was unable to make Five Pebbles cease his increased water intake in due time, and her structure ended up collapsing, leaving her in a comatose state.

If the player doesn't ascend, the Hunter dies and grows into a red version of the Rot which can be seen in the Gourmand campaign, available with the Downpour expansion.

[10] Partly inspired by his feelings of foreignness while living as an exchange student in Seoul, South Korea, a core idea in the game's development was to recreate the life of "the rat in Manhattan".

He originally composed a chiptune-style soundtrack with his musician partner Lydia Esrig, but turned to field recordings of litter for otherworldly sounds.

Primate wanted the music to approximate the game's eclectic visuals, which mix industrial, science fiction, jungle, and various architectural elements.

[21] The early game sound is primitive and based on the slugcat's feelings of fear and hunger, and eventually builds to describe new areas.

[3] Previews compared Rain World to predecessors, including the difficulty of Super Meat Boy, the soundtrack of Fez,[10] and the puzzle-platforming to Metroid and Oddworld.

Named "Rain World: Downpour", it contains five new slugcat characters with their own campaigns, over 1000 new rooms across ten new regions, and three new game modes.

[30] On March 28, 2024, the development of a second DLC titled "Rain World: The Watcher" was announced, featuring multiple new regions, creatures, and a new playable slugcat.

[34][35] Reviewers praised the game's art design and criticized the harshness of its gameplay mechanics,[4][3][8][7] particularly its unpredictable deaths, ruthless enemies, and time-consuming hibernation requirements.

[38] Rock, Paper, Shotgun called the game's checkpointing among the worst in modern platformers, and its challenge, unlike the similarly punishing Dark Souls, without purpose.

[4] PC Gamer's reviewer, with time, began to see Rain World's cumbersome controls less as "bad design" than as "thematically appropriate", given the game's intent to disempower the player.

[7] Rain World was abundant with opportunities for a player to demonstrate ingenuity and improvisation, according to GameSpot's reviewer, whose highlights included making a mouse into a dark room's lantern, using weapons as climbable objects, and luring enemies into battle to distract from the slugcat's presence.

[8] During development, Rain World animations became popular on social media for their "uncanny fluidity",[4] which reviewers continued to praise at release.

[4][37] IGN described slugcat's animations as beautiful and reactive to the angle and physics of movement, from clinging to poles to squeezing through ventilation.

[4] The graphics were more interesting than beautiful to Polygon's reviewer, who also praised the limited color palette's role in distinguishing the slugcat, prey, and enemies from the environment.

[3] While some journalists compared the game's aesthetic to that of Limbo, Rock, Paper, Shotgun's reviewer felt that Rain World had more in common with Oddworld: Abe's Oddysee's aesthetic: both featured similarly dark yet attractive worlds, scary yet fascinating characters, frequent inter-enemy conflict, and frustrating or masochistic controls.

Its imaginative and compelling landscape—surreal inhabitants in a bleak, alien atmosphere—recalled the spirit of games like BioShock and Abzû, in which the reviewer was too attracted to the artistic detail to contemplate the credulity of the man-made environment.