This mode is a reference to the Slender Man Internet horror myths. Press Z to activate this mode. It cannot be deactivated until the game is reset or the day ends. Once pressed, Ayano Aishi will resemble the Slender Man. Ayano'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.
In Slender Mode, Ayano's sanity is set to 0%. Because of this, if Senpai sees her insane, it will be an automatic Game Over. If a teacher sees her, she will be sent to the counselor. Ayano's reputation will be damaged if any students see her. She is able to increase her sanity by laughing insanely or standing in Senpai's pink aura. Any object that Ayano can carry will grow larger, and any squatting or crawling animations will be in the air.
- If Ayano takes off the gloves she is wearing, she will wear the normal school uniform.
- It is extremely difficult to take a picture in this mode, because her hair is flying outwards, blocking the camera.
- If Ayano is in the Art Club and changes into her apron, her skin will be back to normal, as well as her eyes and face. If Ayano then changes back, she will be in her school uniform.
- Before the February 1st, 2016 Build, Ayano 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 Ayano'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".
- The music that plays is called "House of Leaves". It is no longer on the website.