The Matsumoto sarin attack was an attempted assassination perpetrated by members of the Aum Shinrikyo doomsday cult in Matsumoto, Nagano Prefecture, Japan on the night of June 27, 1994. Eight people were killed and over 500 were harmed by sarin aerosol that was released from a converted refrigerator truck in the Kaichi Heights area. The attack was perpetrated nine months before the better-known Tokyo subway sarin attack.

Aleph (Japanese: アレフ, Hepburn: Arefu), formerly Aum Shinrikyo (オウム真理教, Oumu Shinrikyō, literally 'Supreme Truth'), is a Japanese doomsday cult founded by Shoko Asahara in 1987. It carried out the deadly Tokyo subway sarin attack in 1995 and was found to have been responsible for the Matsumoto sarin attack the previous year.

The group says that those who carried out attacks did so secretly, without being known to other executives and ordinary believers. Asahara insisted on his innocence in a radio broadcast relayed from Russia and directed toward Japan.On 6 July 2018, after exhausting all appeals, Asahara and six followers were executed as a punishment for the 1995 attacks and other crimes, and the remaining six on death row were executed on 26 July. At 12:10 am, on New Year's Day 2019, at least nine people were injured (one seriously) when a car was deliberately driven into crowds celebrating the new year on Takeshita Street in Tokyo. Local police reported the arrest of Kazuhiro Kusakabe, the suspected driver, who allegedly admitted to intentionally ramming his vehicle into crowds to protest his opposition to the death penalty, specifically in retaliation for the execution of the aforementioned Aum cult members.

Aum Shinrikyo, which split into Aleph and Hikari no Wa in 2007, had already been formally designated a terrorist organization by several countries, including Russia, Canada, Kazakhstan, the United States, as well as the European Union.The Public Security Intelligence Agency considered Aleph and Hikari no Wa to be branches of a "dangerous religion" and it announced in January 2015 that they would remain under surveillance for three more years. The Tokyo District Court canceled the extension to surveillance of Hikari no Wa in 2017 following legal challenges from the group, but continued to keep Aleph under watch. The government appealed the cancellation, and in February 2019, the Tokyo High Court overturned the lower court's decision, reinstating the surveillance, citing no major changes between Aum Shinrikyo and Hikari no Wa.