Ko no Naishi, historically known as Koshikibu no Naishi, was an early eleventh-century Japanese waka poet. Daughter of the renowned poet Izumi Shikibu, this maternal lineage profoundly shaped her life and poetic standing. From around 1009, she served Empress Shōshi at the imperial court as a lady-in-waiting alongside her mother, positioning her within influential literary and social circles.

Public speculation often questioned her independence from her mother's literary talent. This led to an incident where courtier Fujiwara no Sadayori mocked her before a poetry contest, suggesting she must have sought ghostwritten verses from her distant mother. In response, she composed a poem demonstrating her own skill, referencing the geographical barriers of Ōe-yama, Ikuno, and Ama-no-Hashidate to metaphorically convey her inability to receive maternal guidance while asserting her poetic voice. This defining episode showcased her ability to counter criticism with wit and artistry.

Within the Heian court's social dynamics, she pursued multiple romantic relationships. She eventually married the nobleman Fujiwara no Kiminari, bearing one child. Her life ended in her late twenties shortly after childbirth.

Her legacy endures primarily through her poem's inclusion as the 60th entry in Fujiwara no Teika's Ogura Hyakunin Isshu anthology, cementing her status among celebrated classical poets. The anecdote involving Sadayori and her poem became a recurring subject in later poetic treatises, literary commentaries, and visual art, including Edo-period woodblock prints depicting the court scene where she defended her creative autonomy.