This mode is a reference to the video game Undertale. Press U to activate the mode. It cannot be deactivated until the game is reset or the day ends. Yandere-chan will resemble the character Sans. She will have light grey skin with black eye sockets. She will be bald, and when any special attacks are used, her left eye will glow blue. She will wear a blue jacket and black pants with white stripes. When idle, she will place her hands in her sweatshirt pockets and sway side to side.
Until the February 15th, 2016 Build, the song "Megalovania" from Undertale would play the entire time. This was replaced with the theme of Monty on the Run. In the June 15th, 2016 Build, this mode was finally given an original song in order to avoid copyright infringement. In the July 8th, 2016 Build, the song was replaced with a new original song called Bad Time, created by IRISHTRISH.
If the player presses they Left Control key, a large, six eyed skull appear and shoot a white ray that will kill anything in its way. Pressing Left Control a second time will make two blasters appear, pressing it a third time will make three appear, and so on until the fifth blaster, which will reset the amount to one.
They were originally modeled after Gaster Blasters, but were changed by the request of Toby Fox.
It is impossible to interact with any student in this mode as the button for Psychokinesis replaces the talk or attack button. Once close enough to a student or teacher, the player can pick up them and throw them around. Students who witness the floating corpse will treat it as a random corpse, so no Heroic or Strict teachers will come to try an subdue Yandere-chan. Since Senpai has his pink aura surrounding him, it is not possible to use psychokinesis on him.
After a student dies due to psychokinesis, it is not possible to use it again on the corpse.
If the player holds down the Left Control key, many bones will rise from the ground within a short range. Any student or teacher that comes within contact with a bone will die instantly. The bones will appear through any prop, including walls.
- Yandere-chan's textures will glitch if she wears gloves. If the player takes off the gloves, then Yandere-chan will have the normal school uniform.
- Yandere-chan will lose her skin texture if she goes nude in the Shower Room. If she puts on a uniform, she will revert back to Yandere-chan, but with her powers still activated.
- If Yandere-chan uses the bone attack while Kokona is eating on Wednesday, she will die. If the player picks up her body, she will continue her eating animation while Yandere-chan is carrying her.
- If the player uses psychokinesis in front of the delinquents, they will run up and hit her with their weapons, but she will not fall into a coma. Once she drops the student, the delinquent will continue to follow her around without harming Yandere-chan.
- If Senpai is killed in this mode and dismembered, Yandere-chan can carry one of his body parts and walk out of the school into a small part of the town.
- This mode was implemented in the January 15th, 2016 Build.
- All Undertale audio, Sans' battle song MEGALOVANIA, and the original Gaster Blasters were removed in the February 15th, 2016 Build.
- Kyoshi Taiso borrows the same track suit and hip sway animations.
- If Senpai is killed in this mode, he will respawn the next day.
- The player can press D to disable the copyrighted music.
- YandereDev has stated that Osana Najimi will perform the same hip swaying animation as "Sansdere-chan", the nickname he gives to Yandere-chan during this mode.