Sei Shōnagon (清少納言), (c.966-1017) was a Japanese author and a court lady who served the Empress Teishi around the year 1000 during the middle Heian Period, and is best known as the author of The Pillow Book (枕草子 makura no sōshi).

Sei Shōnagon's actual given name is not known. It was the custom among aristocrats in those days to call a court lady (女房nyōbō) by a combined appellative taken from a) her clan name and b) some court office belonging either to her or some close relative. Sei (清) thus derives from the Kiyohara (清原) clan, while Shōnagon was a government post. It is unknown which of her relatives held the post of shōnagon. Her actual name has been a topic of debate among scholars, who generally favor Kiyohara Nagiko (清原 諾子) as a likely possibility.

Little is known about her life except what is said in her writings. She was the daughter of Kiyohara no Motosuke, a scholar and famous waka poet. Her grandfather Kiyohara no Fukayabu was also a well-known waka poet. They were middle-ranking courtiers and had difficulties, since they were never granted a revenue-producing court office.

She married Tachibana Norimitsu at the age of 16, and gave birth to one son. Afterwards, she remarried to Fujiwara-Muneyo, and gave birth to the daughter. At age 27, when she began to serve the Empress Teishi, consort of Emperor Ichijō, she was supposedly divorced. She was fascinated by the young and beautiful Empress, a teenager when they met.

Shōnagon achieved fame through her work, The Pillow Book, a collection of lists, gossip, poetry, observations, complaints, and anything else she found of interest during her years in the court. In The Pillow Book Shōnagon reports the troubles Empress Teishi had after her father died and Regent Fujiwara no Michinaga made one of his daughters another consort of the Emperor Ichijō. Because of the risk of fire, the Imperial family did not, at that time, live in the Heian Palace. Empress Teishi resided in a part of Chūgushiki, the Bureau of Serving the (Middle) Empress. Sei refers to the death of her patroness, who died in childbirth in 1000, with refined lightheartedness and implies it was not difficult. To do otherwise would have been considered unstylish. Her writing depicts the court of the young Empress as full of an elegant and merry atmosphere.

There are no details about Shōnagon's life after the death of the Empress, though The Pillow Book is thought to have been finished sometime between 1001 and 1010. One story has Shōnagon living out her twilight years in poverty, but this is probably a legend spread by those who disapproved of her alleged promiscuity.

Shōnagon is also known for her rivalry with her contemporary writer and court lady Murasaki Shikibu, who wrote The Tale of Genji and served the Empress Shoshi, second consort of the Emperor Ichijō. Murasaki Shikibu wrote about Shōnagon in her diary, The Murasaki Shikibu Diary.