This mode is a reference to the Slender Man Internet horror mythos. Press Z to activate this mode. It cannot be deactivated until the game is reset or the day ends. Once pressed, Yandere-chan will resemble the Slender Man. Yandere-chan's limbs will be extremely elongated, her eyes become fully dull, a shadow will appear over her face, and her uniform, skin, and background will gain a monochrome scheme. A suspenseful song, "House of Leaves" by Kevin MacLeod, will play the entire time.
In Slender mode, the player's sanity is set to minimum. Because of this, if Senpai or a teacher see Yandere-chan insane, it will be an automatic Game Over. The player's reputation will be damaged if any students see her. They are able to increase their sanity by laughing insanely or standing in Senpai's pink aura. Any object that the player can carry will grow larger, and any squatting or crawling animations will be in the air.
- If the player takes off the gloves they are wearing, Yandere-chan will wear the normal school uniform.
- Before the February 1st, 2016 Build, Yandere-chan would have a white face with no visible eyes. After this build, she will have visible eyes and a dark shadow on her face, representing little sanity.
- Slender Mode was implemented in the May 19th, 2015 Build.
- This mode was created because it was very easy to stretch out Yandere-chan's limbs.
- This mode shares the same music as X Mode.
- The game's warning referred to this mode, saying that this game contained "True Nightmares". This warning was later removed because YandereDev said that it was "cringey".