The player controls a child who has fallen into the Underground: a large, secluded region under the surface of the Earth, separated by a magical barrier.

The game was acclaimed for its thematic material, intuitive combat system, musical score, originality, story, dialogue, and characters; however, the reaction to its art style was mixed.

During battles, players control a small heart which represents their soul, and must avoid attacks unleashed by the opposing monster similar to a bullet hell shooter.

[15] A human child falls into the Underground from Mount Ebott and encounters Flowey, a sentient flower who teaches the player the game's mechanics and lies about "LV", or "LOVE", in an attempt to kill them.

They then receive a phone call from Sans, detailing the state of the Underground after the human's departure, with the specifics depending on who was killed or spared.

To enable further replays of the game, the player must give their soul to Chara, restoring the world and causing a permanent alteration to the Pacifist ending.

[23] He set out to develop a game with "interesting characters", and that "utilizes the medium as a storytelling device ... instead of having the story and gameplay abstractions be completely separate".

He found that "there's a psychological thread that says audiences become more attached to characters drawn simply rather than in detail", particularly benefiting from the use of visual gags within the art.

[29] Retrospectively, Fox described Undertale as "a huge katamari of things that [he likes] that [he] combined arbitrarily," adding that part of the story "was almost written improvisationally at the last minute".

[32] The game's dialogue system was inspired by Shin Megami Tensei (1992),[30] particularly the gameplay mechanic whereby players can talk to monsters to avoid conflict.

[33] Despite not having played it, Fox was inspired by the concepts of Moon: Remix RPG Adventure (1997), which involved the player repairing the damage of the "Hero" and increasing their "Love Level" by helping people instead of hurting them.

Fox strongly disliked the use of the companion character Fi in The Legend of Zelda: Skyward Sword, in which the answers to puzzles were often revealed early.

Papyrus in particular was conceived as a sketch in Fox's notebook; he was originally a mean spirited character named "Times New Roman" who wears a fedora.

As part of the fifth anniversary of the game, Fox streamed footage with permission of a 2019 concert of the Undertale songs performed by Music Engine, an orchestra group in Japan, with support of Fangamer and 8-4.

[66] Another album of jazz duets based on Undertale's songs, Prescription for Sleep, was performed and released in 2016 by saxophonist Norihiko Hibino and pianist Ayaki Sato.

[74][75] To ensure a consistent voice for the game, 8-4 chose to employ only one primary translator, Keiko Fuchicho, even though it would lead to the Japanese version taking longer to release.

Before the translation began in early 2016, Fox provided development notes and annotated dialogue to the team, and he continued to support and advise them throughout its creation.

GameSpot's Tyler Hicks declared it "one of the most progressive and innovative RPGs to come in a long time",[46] and IGN's Kallie Plagge called it "a masterfully crafted experience".

[17] Jesse Singal of The Boston Globe found the game's ability to make the player empathize with the monsters during combat if they opted for non-violent actions was "indicative of the broader, fundamental sweetness at the core" of Undertale.

[92] Essayist Sean Travers in the Journal of Popular Culture described Undertale as a critique of the violent nature of mainstream role-playing video games, where players kill enemies in exchange for experience points.

She noted the game's rather punitive nature towards those who choose to play through the Genocide route and ignore warnings, citing its progressive change in tone and disturbing imagery.

"[93] Kevin Vale of Convergence compared Undertale's management of accountability to that of Night in the Woods, citing the central role the player has in them, while contrasting their use of gameplay elements.

[94] Reviewers praised the game's writing and narrative, which IGN's Plagge labeled it as "strong" and "consistently funny" and noted how it "[built] on" Undertale's themes of humanity and morality.

[104][105][106] Professional wrestler Kenny Omega has expressed his love of Undertale, dressing as Sans for the October 30, 2019, episode of All Elite Wrestling: Dynamite.

Ultimate garnered positive feedback from fans, although The Commonwealth Times considered his addition to be a "potential problem" due to the decreasing nostalgia factor for each new character and ever-increasing size of the roster.

[109] Among the most anticipated fangames was the spinoff Undertale Yellow, which after seven years of development was released in 2023 and met with immense popularity by fans on multiple social media networks.

[111] In July 2016, during a summit about the Internet held at the Vatican, YouTube personality MatPat gifted a copy of Undertale to Pope Francis.

A commonly proposed theory by the fandom is that Chara, the first human who fell into the underground and is credited to causing the destruction inflicted in the Genocide route, is the narrator of the game.

The Gaster story has very few pieces of evidence confirmed to have been known by the community outside of his association with the font Wingdings, his role as the royal scientist, and creating the CORE (a magical and technological power source for the Underground's monsters who live in the most populated areas) within Undertale only to "fall into his creation" and have his body shattered across time and space.

[134][135] After previously teasing something Undertale-related a day earlier, Fox released the first chapter of Deltarune on October 31, 2018, for macOS and Windows for free.