Diary of a Madman may refer to:
Films- Diary of a Madman (film), a 1963 horror film starring Vincent Price
- Diary of a Madman (1990 film), directed by Ronan O'Leary, cinematography by Walter Lassally
- Diary of a Madman (Nikolai Gogol), a short story by Nikolai Gogol
- Diary of a Madman (Guy de Maupassant), a short story by Guy de Maupassant
- Diary of a Madman (Lu Xun), a short story by Lu Xun, also known as A Madman's Diary
- Diary of a Lunatic, a short story by Leo Tolstoy sometimes translated as "The Diary of a Madman"
- Diary of a Mad Old Man, a novel by Jun'ichirō Tanizaki
- Diary of a Madman, a memoir by rapper Scarface
- Diary of a Madman (album), by Ozzy Osbourne, or the title track
- "Diary of a Madman" (song), by Gravediggaz
- The Diary of a Madman (opera), a 1958 chamber opera by Humphrey Searle
- Diary of a Maniac, 1993 Italian drama film
- Diary of a Mad Old Man, 1961 novel by Japanese author Jun'ichirō Tanizaki