"The Moon over the Ruined Castle" ("Kōjō no Tsuki") is a Japanese song written in the Meiji period. Japanese pianist and composer Rentarō Taki composed the music as a music lesson song without instrumental accompaniment in 1901. 

A jazz arrangement was recorded by Thelonious Monk under the title "Japanese Folk Song" on his 1967 album Straight, No Chaser. This version can be heard in the movie La La Land, as one of the main characters tries to memorize and play it.