Undertale quickly developed a cult following.About a year after release, Fox commented that he was surprised by how popular the game had become and though appreciative of the attention, he found it stressful. Fox said: "It wouldn't surprise me if I never made a game as successful again. That's fine with me though". Ana Diaz of NPR stated that Undertale "shaped the entertainment tastes of an entire generation", and that it "took on a new life through memes".Reid Young, CEO of Fangamer, said that the game became "an absolutely dominating force".The character Sans has been well received by players, being the subject of many fan works.Professional wrestler Kenny Omega has expressed his love of Undertale, dressing as Sans for the October 30, 2019, episode of All Elite Wrestling: Dynamite.Sans's addition as a Mii fighter costume in Super Smash Bros. 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.
Undertale's fanbase has also been subject to controversy, leading to it gaining a negative reputation. Part of it was due to the disagreement between fans over the correct way to play the game.After the game's release, some live streamers were harassed by fans of the game for killing in-game enemies and attempting the "genocide" route. YouTuber Markiplier refused to complete his initial play-through of the game, stating that he was "not having fun" due to fan demands.
In July 2016, during a summit about the Internet held at the Vatican, YouTube personality MatPat gifted a copy of Undertale to Pope Francis. MatPat explained his choice of gift by referencing the year 2016's status as the Extraordinary Jubilee of Mercy, and connecting this to Undertale's overarching theme of mercy.Later, in January 2022, a circus troupe performed in front of the Pope during his weekly audience at the Vatican to the tune of "Megalovania", drawing parallels with MatPat's symbolic gift of the game to Pope Francis.