Events from the year 1921 in Japan. It corresponds to Taishō 10 (大正10年) in the Japanese calendar.

• Emperor: Taishō^[1]
• Regent: Hirohito (from November 25)
• Prime Minister:
  • Hara Takashi (until November 4)
  • Takahashi Korekiyo (from November 13)

• Aichi Prefecture: Shunji Miyao (until 18 April); Hikoji Kawaguchi (starting 18 April)
• Akita Prefecture: Ryoshin Nao
• Aomori Prefecture:
  • until 9 March: Hidehiko Michioka
  • 9 March-28 September: Shundo Kahei
  • starting 28 September: Yujiro Ozaki
• Ehime Prefecture: Toshio Mawatari (until 27 May); Juunosuke Miyazaki (starting 27 May)
• Fukui Prefecture: Kohei Yuji (until 27 May); Josuke Shiraogawa (starting 27 May)
• Fukuoka Prefecture: Yasukouchi Asakichi
• Fukushima Prefecture: Miyata Mitsuo
• Gifu Prefecture: Kanokogi Kogoro (until 27 May); Manpei Ueda (starting 27 May)
• Gunma Prefecture: Muneyoshi Oshiba
• Hiroshima Prefecture: Raizo Wakabayashi (until 19 July); Ichiro Yoda (starting 19 July)
• Ibaraki Prefecture: Yuichiro Chikaraishi (until 27 May); Genjiro Moriya (starting 27 May)
• Iwate Prefecture: Takeo Kakinuma
• Kagawa Prefecture: Yoshibumi Satake
• Kochi Prefecture: Abe Yoshihiko
• Kumamoto Prefecture: Hikoji Kawaguchi (until 27 May); Sansuke Nakayama (starting 27 May)
• Kyoto Prefecture: Eitaro Mabuchi (until July); Raizo Wakabayashi (starting July)
• Mie Prefecture: Haruki Yamawaki
• Miyagi Prefecture: Mori Masataka (until 27 May); Yuichiro Chikaraishi (starting 27 May)
• Miyazaki Prefecture: Naomiki Hirose (until 3 July); Goro Sugiyama (starting 3 July)
• Nagano Prefecture: Tenta Akaboshi (until 27 May); Tadahiko Okada (starting 27 May)
• Niigata Prefecture: Ota Masahiro
• Okayama Prefecture: Masao Kishimoto
• Okinawa Prefecture: Sōsuke Kawagoe (until 27 May); Jyun Wada (starting 27 May)
• Saga Prefecture: Sawada Ushimaro (until 3 June); Tominaga (starting 3 June)
• Saitama Prefecture: Horiuchi Hidetaro
• Shiname Prefecture: Sanehide Takarabe
• Tochigi Prefecture: Hiroyoshi Hiratsuka
• Tokushima Prefecture: Rinpei Otsu (until month unknown)
• Tokyo: Hiroshi Abe (until 27 May); Katsuo Usami (starting 27 May)
• Toyama Prefecture: Higashizono Motomitsu (until 24 December); Shida Jisho (starting 24 December)
• Yamagata Prefecture: Ichiro Yoda (until 19 July); Morimoto Izumi (starting 19 July)
• Yamanashi Prefecture: Miki Nagano

• January 15 – Mitsubishi Electronics was separate section from Mitsubishi Shipbuildings (now Mitsubishi Heavy Industries) which was founded in Ōsone, Nagoya.^{[citation needed]}
• January Unknown date – Komatsu Limited was founded, as predecessor name was Komatsu Ironworks.^{[page needed]}
• September – Sempill Mission sent by Britain to Japan, with the objective of helping the Imperial Japanese Navy develop its aeronaval forces.
• October 16 – Janome Sewing Machine founded.^{[page needed]}
• November 4 – Prime Minister Hara Takashi is assassinated at Tokyo Station.
• November 12 – Washington Naval Conference opens at Memorial Continental Hall Hall in downtown Washington DC.
• November 25 – Crown Prince Hirohito is made regent in place of his ailing father.
• December 13 – In the Four-Power Treaty on Insular Possessions, Japan, the United States, United Kingdom, and France agree to recognize the status quo in the Pacific.
• Termination of Anglo-Japanese Alliance.
• Ladies' Agreement
• Unknown date – Otsuka Pharmaceutical (大塚製薬) founded in Naruto, Shikoku Island.^{[page needed]}

• January 3 – Natsuko Kahara, actress (d. 1991)
• January 26 – Akio Morita, businessman and co-founder of Sony Corporation (d. 1999)
• February 9 – Junzo Shono, writer (d. 2009)
• February 11 – Edward Seidensticker, scholar, historian, and translator (d. 2007)
• February 14 – Toshiko Taira, textile artist (d. 2022)
• March 19 – Heitaro Nakajima, engineer (d. 2017)
• April 20 – Michiko Inukai, writer and philanthropist (d. 2017)
• December 20 – Kosuke Gomi, novelist (d. 1980)

• January 13 – Ijuin Gorō, admiral (b. 1852)
• June 19 – Nabeshima Naohiro, politician and former daimyō (b. 1846)
• September 28 – Yasuda Zenjirō, entrepreneur, founder of the Yasuda zaibatsu (b. 1838)
• November 4 – Hara Takashi, Prime Minister of Japan (b. 1856)
• November 5 – Yusuke Hashiba, archaeologist, historian and anthropologist (b. 1851)
• December 29 – Hayashi Yūzō, politician (b. 1842)

• List of Japanese films of the 1920s