Deltarune is an episodic role-playing video game by American indie developer Toby Fox. Set in a parallel universe to his 2015 game, Undertale, the game follows Kris, a human teenager predetermined to save the world alongside Susie, a monster, and Ralsei, a prince from the Dark World. Throughout the story, the party establishes relationships with a variety of allies and opponents.

The game's combat system is turn-based and uses bullet hell mechanics. Like in Undertale, enemy encounters can be resolved peacefully or through violence. The game shares several characters and gameplay elements with Undertale, but takes place in a separate setting and features an overhauled battle system with multiple party members.

Development for Deltarune began in 2012. Its first two chapters were released as a free demo in 2018 and 2021 respectively, while the third and fourth chapters were made available in 2025 with its paid release. Initially released for macOS and Windows, it was later ported to the Nintendo Switch and PlayStation 4 in 2019, followed by the Nintendo Switch 2 and PlayStation 5 in 2025. The game has been praised by critics for its soundtrack, characters, narrative, and sense of humor.

Deltarune is a role-playing video game with a top-down perspective. The player controls a human named Kris Dreemurr and has a party of other characters who they select actions for during battle. The game features both overworld and combat sections. In the overworld, the player can solve puzzles, collect items, and interact with characters and the environment. Unlike Undertale, Deltarune does not have a random encounter system. Instead, players can see the enemies in the environment before they encounter them, allowing for the possibility of avoiding or deliberately initiating combat.

The game uses a turn-based combat system, where players can choose from a set of actions each turn: fight, act, item, spare, and defend. As in Undertale, players control a heart-shaped soul in a fixed area and must dodge enemy attacks in the form of bullets, though the exact mechanics are sometimes unique to the respective enemy. Defending or "grazing" an incoming attack by letting it come close to the heart without getting hit by it increases the tension points (TP) gauge, which allows party members to use spells or particular acts. For example, Ralsei can pacify tired enemies with a spell.

Using acts and spells allows a player to spare enemies non-violently; from the second chapter, this can allow them to be recruited to live in the player's Castle Town. Alternatively, fighting an enemy, which also increases TP, can lead to them being defeated violently.

Deltarune's narrative spans multiple chapters, of which four have been released, with three more being planned for a later release (of which two have been publicly disclosed to be in active development). Although it features characters and elements from Undertale, it takes place in a different setting; it is officially referred to as a "parallel story" to that game.

The game begins with a voice prompting the player to create an avatar, but the "vessel" is ultimately discarded, as "no one can choose who they are in this world". Instead, the player is given control of human teenager Kris, who arrives to school late. After being paired up with their delinquent monster classmate Susie for a group project, their teacher, Alphys, sends the two to get supplies. However, the supply closet shuts behind them and they fall into a strange realm—the Dark World—where they meet the dark prince Ralsei. He tells them of a prophecy stating that the three of them are heroes destined to close the geyser-like Dark Fountains that give form to the Dark Worlds. A new Dark Fountain has created a Dark World next to Ralsei's, ruled by the tyrannical King. Susie, uninterested in Ralsei's prophecy, joins King's son Lancer in fighting Kris and Ralsei, but both are convinced to change sides and join the party.