The Tosa Nikki is a poetic diary written anonymously by the tenth-century Japanese poet Ki no Tsurayuki. The text details a 55-day journey in 935 returning to Kyoto from Tosa province, where Tsurayuki had been the provincial governor. The prose account of the journey is punctuated by Japanese poems, purported to have been composed on the spot by the characters.

Poetic diary or Nikki bungaku (日記文学) is a Japanese literary genre, dating back to Ki no Tsurayuki's Tosa Nikki, compiled in roughly 935. Nikki bungaku is a genre including prominent works such as the Tosa Nikki, Kagerō Nikki, and Murasaki Shikibu Nikki. While diaries began as records imitating daily logs kept by Chinese government officials, private and literary diaries emerged and flourished during the Heian period.