Otsuichi (乙一, Otsuichi) is the pen name of Hirotaka Adachi (安達 寛高, Adachi Hirotaka), born 1978. He is a Japanese writer, mostly of horror short stories, as well as a filmmaker. He is a member of the Mystery Writers of Japan and the Honkaku Mystery Writers Club of Japan.

He made his debut with Summer, Fireworks and My Corpse while still in high school. Major works include the novel Goth, which was adapted into a comic and a feature film (Goth: Love of Death) and the Zoo short story collections which were also adapted into a feature film. Goth won the 2003 Honkaku Mystery Award.

Tokyopop has published English-language translations of his short story collection Calling You, the novel Goth and the comic adaptations of both. Another short story, F-Sensei's Pocket, appears in the English edition of the literary magazine Faust.