Undyne is a fictional character in the 2015 video game Undertale and episodic video game Deltarune. She is a monster who serves as the captain of the Royal Guard of the Underground under its king, Asgore. She is initially an antagonist to the player character, but depending on the human's actions, she may become their ally. She is reluctant to befriend the human, but through persistence by the player, eventually realizes that they were a good person. If the human makes a point of killing every monster possible, Undyne will transform into Undyne the Undying upon death, vowing to destroy the human in order to save humans, monsters, and all other life from their actions. She is a lesbian, revealed to be in love with Alphys.

Undyne was created by Toby Fox, who created most of the game. He received assistance from other artists, including Temmie Chang. Undyne went through multiple depictions in early concept designs, including being more fish-like in art by Fox and resembling an Axem Ranger from Super Mario RPG in art by Temmie. She was difficult for Fox to design at first before Fox settled on her final design and characterization. Undyne later appeared in Deltarune, though in an unrelated role to the one she has in Undertale, as she was now depicted as the chief of police of Deltarune's hometown in the light world, has no connection to Alphys, and does not wear an eyepatch.

Since her appearance in Undertale, she has been met with positive reception. Undyne's sexuality, as well as her relationship with Alphys, were praised by critics for how they were depicted. She was considered one of the best characters in and out of Undertale, particularly as a female character. Her boss fights were also praised, with Game Informer citing the "genocide" route boss fight against her as one of the game's best. Her music was also discussed, with critics and writers discussing how her music reflects her personality and actions. She has been the recipient of multiple pieces of merchandise, including a Nendoroid figure from Good Smile and multiple pieces from Fangamer.

Undyne was created by Toby Fox for the video game Undertale, and was also designed by others involved, such as Temmie Chang. According to initial concept designs, Undyne was originally meant to look more fish-like. Other early concept designs created by Temmie were meant to look like an Axem Ranger's head from Super Mario RPG per Fox's vision. Undyne is a lesbian monster who serves as the captain of the Royal Guard of the Underground under its king, Asgore, tasked with getting a human's soul to free the monsters in the Underground. She is first seen in a full suit of armor, though when her helmet is removed, she is shown to be a fish woman with red hair pulled back in a ponytail, blue skin, fangs, and only one eye. She fights with a blue spear, and can summon energy spears as well. When not in her armor, she is seen wearing a tank top and pants.

She was difficult for Fox to decide on a final design, both in terms of appearance and characterization, with the only consistent detail being that she would be the first boss to try to kill the protagonist. He tried various accents and hobbies, such as fashion and singing, but was not satisfied. The final concept only materialized for Fox once he began writing the monologue she spoke before her boss battle with the protagonist. At this point, he found her natural to write. Her face and body were also difficult to settle on, taking a long time to pick a final design. Her first sprite by Temmie was deemed too much like a "normal knight" by Fox, who addressed this by tweaking her helmet. Other changes he made included making her waist-armor longer to avoid looking like a miniskirt and changing the lighting of the sprite to make her more intimidating.

When designing a "cute outfit" for Undyne, artist Gigi commented that it looked too much like jogging clothes. A friend suggested adding a jacket, which Fox did. When designing Undyne the Undying, an alternate form Undyne may assume, Fox intended for her hair to be more vertical, but found that it would not fit on screen. Fox originally considered giving her a pet fish, but found it strange since Undyne was also a fish. Her design was first revealed through a Steam digital trading card shortly before the release date illustrated by Gigi. Toby Fox intended to make it clearer what players had to do in the fight against Undyne due to multiple players having trouble understanding that they had to run away. He added an extra line to her dialogue to telegraph to players that they needed to run away.

When composing the theme in Undyne's first boss battle, Fox used the same trumpet sounds used by ZUN in the Touhou series. Her home was designed by artist Gigi, with the only notes given by Fox being that it needed to look funny when it catches on fire. Undyne had another song that was planned before being scrapped, which was the most complete of all scrapped songs in Undertale. It was made before Fox fully understood Undyne's personality, and despite him liking the arrangement, he felt it did not sound enough like "being heroically punched in the face."

In Undertale, she appears as an antagonist to the protagonist, the human, attempting to kill them in order to extract their soul and use it to free the monsters from the Underground to fulfill Asgore's plans. She is also friends with Alphys, whom she met in the dump. She pursues them throughout Waterfall, and eventually fights them in a duel. The human can either fight Undyne or run away, but cannot spare her. If they run away enough, they are able to escape. Under the right circumstances, Undyne can be befriended through an unsuccessful cooking date orchestrated by Papyrus, where Undyne realizes that the human has a kind heart. Depending on the player's actions before fighting Asgore, Undyne's ending will change.