Undertale is a 2015 role-playing video game created by American indie developer Toby Fox. 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 player meets various monsters during the journey back to the surface, some of which may engage in combat. The combat system involves the player navigating through mini–bullet hell attacks by the opponent. They can opt to appease monsters in order to spare them instead of killing them. These choices affect the game, with the dialogue, characters, and story changing based on outcomes.

Outside of artwork and character designs by Temmie Chang and other guest designers, Fox developed the entirety of the game by himself, including the script and music. The game took inspiration from several sources, including the Brandish, Mario & Luigi, and Mother role-playing game series, bullet hell shooter series Touhou Project, role-playing game Moon: Remix RPG Adventure, and British comedy show Mr. Bean. Undertale was originally meant to be two hours in length and was set to be released in mid-2014, but experienced delays.

Undertale was released for OS X and Windows in September 2015. It was ported to Linux in 2016, PlayStation 4 and PlayStation Vita in 2017, the Nintendo Switch in 2018, and Xbox One in 2021. The game was acclaimed for its thematic material, combat system, musical score, originality, story, dialogue, and characters, although reaction to the art style was mixed. It has sold at least five million copies. It was nominated for multiple accolades, including game of the year from several gaming publications and conventions, and is considered one of the greatest video games ever made. A game with a parallel story, Deltarune, released episodically from 2018 before its full 2025 launch; further chapters are in development.

Undertale is a role-playing video game that uses a top-down perspective. In the game, the player controls a child and completes objectives in order to progress through the story. Players explore an underground world filled with towns and caves, and are required to solve numerous puzzles on their journey. The underground world is the home of monsters, many of whom challenge the player in combat; players decide whether to kill, flee from, or befriend them. Choices made by the player radically affect the plot and general progression of the game, with the player's morality acting as the cornerstone for the game's development.

When players encounter enemies in either scripted events or random encounters, they enter a battle mode. 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. As the game progresses, new elements are introduced, such as colored obstacles, and boss battles which change the way players control the heart. Players may choose to attack the enemy, which involves timed button presses. Killing enemies will cause the player to earn EXP (in turn increasing their LOVE) and gold. They can use the ACT option to check an enemy's attacking and defending attributes as well as perform various other actions, which vary depending on the enemy. If the player uses the right actions to respond to the enemy, or attacks them until they have low HP (but are still alive), they can then choose to spare them and end the fight without killing them. For some boss encounters to be completed peacefully, the player is required to survive until the character they are facing has finished their dialogue. The game features multiple story branches and endings depending on whether players choose to kill or spare their enemies; and as such, it is possible to clear the game without killing a single enemy.

Monsters will talk to the player during the battle, and the game will tell the players what the monster's feelings and actions are. Enemy attacks change based on how players interact with them: should players choose non-violent options, enemy attacks are comparatively easy, whereas they become less easy if players choose violent options. The game relies on a number of metafictional elements in both its gameplay and story. When players participate in a boss battle on a second playthrough, the dialogue will be altered depending on actions in previous playthroughs.

Undertale is set in the Underground, a realm where monsters were banished after war broke out with humans before the events of the game. The Underground is sealed from the surface by a magic barrier with a singular gap at Mount Ebott. At the start of the game a player-named 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. The human is then rescued by Toriel, a motherly goat-like monster, who teaches them how to survive conflict in the Underground without killing. She intends to adopt the human, wanting to protect them from Asgore, the king of the Underground.

The human eventually leaves Toriel to search for Asgore's castle, which contains the barrier to the surface world. They encounter several monsters, such as the skeletons Sans and Papyrus, two brothers who act as sentries for the Snowdin forest; Undyne, the head of the royal guard; Alphys, the kingdom's royal scientist; and Mettaton, a robotic television host Alphys created. Most of the monsters are fought, with the human choosing whether to kill them or to spare and possibly befriend the monster. During their travels, the human learns that many years ago, Asriel, the son of Asgore and Toriel, befriended the first human who fell into the Underground. When the child abruptly died, Asriel used the child's soul to pass through the barrier, intending to return the body to the surface. The humans living there attacked and killed Asriel, causing Asgore to declare war. In the present day, Asgore has collected six souls from fallen humans, and needs one more to destroy the barrier.