Otsuichi (乙一, Otsuichi) is the pen name of Hirotaka Adachi (安達 寛高, Adachi Hirotaka; born October 21, 1978) is a Japanese writer and 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.

Otsuichi was born on October 21, 1978, in Tanushimaru (now part of Kurume), Fukuoka Prefecture, the eldest son of a family of four with his parents and two older sisters. He attended Kawai Elementary School and during his later years there, he became overweight, surpassing 60 kilograms. This led to Otsuichi getting bullied and gaining an inferiority complex, which he used playing video games alone to deal with. After Kawai, he attended Kurume Shiritsu Tanushimaru Junior High School. At the age of 14, he decided to go on a diet, and was elected class president in the third year of junior high school. Despite this, he still felt inferior to his classmates.

In 1994, he entered the National Institute of Technology, Kurume College. Despite calling the 5 years at the college "the gloomiest in all of his life," during summer vacation when he was around 15 years old, he read the first volume of the light novel series Slayers by Hajime Kanzaka which he borrowed from a friend and discovered his love of reading, and began dabbling into the world of light novels and manga. For the next year and a half, Otsuichi read as many light novels as he could get his hands on either from friends or his older sisters, while also encountering mystery novels such as Takemaru Abiko's Satsuriku ni taru Yamai, Yukito Ayatsuji's The Decagon House Murders and Sōji Shimada's Detective Kiyoshi Mitarai series.

He started writing his own novels when he was 16 years old. To qualify for the Fujimi Fantasia Novel Awards, Otsuichi starting writing an isekai fantasy novel, but feeling that it was not going well, he decided to rewrite the setting to a rural town nearby where he grew up. This reworked novel was called Summer, Fireworks and My Corpse and it won the 6th JUMP Novel Grand Prix award in 1996 after a strong endorsement from Kaoru Kurimoto, making his official debut at only 17 years old.

In 1999, he graduated from the National Institute of Technology, Kurume College, and transferred to the Department of Ecology Engineering, Toyohashi University of Technology, and started living on his own in Toyohashi, Aichi Prefecture. At university, he belonged to a science fiction research group.

In 2002, Otsuichi graduated from Toyohashi University. This year, one of his most known work, Goth, was published and the following year, it won the 3rd Honkaku Mystery Award. In 2003, he moved from Toyohashi to the vicinity of Tokyo's Gakugei-daigaku Station, and a few months later to the vicinity of Musashi-Nakahara Station in Kawasaki, Kanagawa Prefecture.