Events in the year 1904 in Japan. It corresponds to Meiji 37 (明治37年) in the Japanese calendar.

• Emperor: Emperor Meiji^[1]
• Prime Minister: Katsura Tarō

• February 8–9 – Battle of Port Arthur: A surprise Japanese naval attack on Port Arthur (Lüshun) in Manchuria starts the Russo-Japanese War.^[2][3][4][5]
• February 9 – Battle of Chemulpo Bay^[6][7][8]
• February 23 – Japan–Korea Treaty of 1904^[9]
• April 30–May 1 – Battle of Yalu River (1904)^{[10][11][12][13][14]}
• May 25–26 – Battle of Nanshan^[15]
• June 14–15 – Battle of Te-li-Ssu^[16]
• July 10 – Battle of Motien Pass
• July 24–25 – Battle of Tashihchiao
• July 31 – Battle of Hsimucheng
• August 10 – Battle of the Yellow Sea
• August 14 – Battle off Ulsan
• August 20 – Battle of Korsakov
• August 22 – Japan–Korea Agreement of August 1904
• August 24–September 4 – Battle of Liaoyang
• October 5–17 – Battle of Shaho
• Unknown date – Hirano Rubber Manufacturing, as predecessor of Toyo Tire was founded.^{[page needed]}

• February 9 – Kikuko Kawakami, author (d. 1985)
• May 27 – Chūhei Nambu, track and field athlete (d. 1997)
• June 1 – Ineko Sata, communist and feminist author of proletarian literature (d. 1998)^[17]
• July 18 – Fuji Yahiro, screenwriter (d. 1986)^[18]
• July 25 – Katsuji Matsumoto, illustrator (d. 1986)
• August 7 – Taro Takemi, physician and 11th President of the Japan Medical Association (d. 1983)
• August 16 – Minoru Genda, military aviator and politician (d. 1989)
• September 1 – Aya Kōda, essayist and novelist (d. 1990)^[19]
• October 11 – Ken'ichi Enomoto, comedian and singer (d. 1970)
• November 18 – Masao Koga, composer (d. 1978)
• November 22 – Fumio Niwa, novelist (d. 2005)
• December 28 – Tatsuo Hori, writer, poet and translator (d. 1953)

• January 1 – Konoe Atsumaro, politician and journalist (b. 1863)^[20]
• January 9 – Ii Naonori, former daimyō, son of Ii Naosuke (b. 1848)
• February 6 – Utagawa Yoshiiku, artist (b. 1833)^[21]
• March 27 – Takeo Hirose, navy career officer (b. 1868)^[22][23]
• August 12 – Kawamura Sumiyoshi, admiral (b. 1836)
• August 31 – Tachibana Shūta, soldier (b. 1865)
• September 26 – Koizumi Yakumo, writer (b. 1850)