A Certain Item of Dark Side (Japanese: とある暗部の少女共棲, Hepburn: Toaru Anbu no Aitemu)[a] is a Japanese light novel series written by Kazuma Kamachi and illustrated by Nilitsu, which is a spin-off of Kamachi's A Certain Magical Index series, focusing on the women belonging to Academy City's dark side.

ASCII Media Works have published three volumes since March 2023 under their Dengeki Bunko imprint.

A manga adaptation with art by strelka has been serialized in ASCII Media Works' shōnen manga magazine Dengeki Daioh since October 2023 and has been collected in a single tankōbon volume.

Written by Kazuma Kamachi and illustrated by Nilitsu, A Certain Item of Dark Side began publication under ASCII Media Works' Dengeki Bunko light novel imprint on March 10, 2023.

[7] An anime television series adaptation was announced during the "Dengeki Bunko Winter Festival 2025" livestream event on February 16, 2025.