?tsu means "big port", because ?tsu was an important port on Lake Biwa since ancient times. In the years 667 to 672, the ?mi ?tsu Palace was founded by Emperor Tenji. The Jinshin War devastated the Omi Otsu Palace, and ?tsu was renamed Furutsu. A new capital, Heian-ky?, (now Kyoto) was established in the immediate neighborhood in 794, and ?tsu was revived as an important traffic point and satellite town of the capital.
In Edo period, ?tsu prospered as a port on Lake Biwa and a shukuba of the Tokaido (?tsu-juku). The Zeze Domain was based in Zeze, a neighboring castle town of ?tsu-juku.
On 11 May 1891, the ?tsu incident, a failed assassination attempt on Tsarevich Nicholas Alexandrovich of Russia (later Tsar Nicholas II), occurred.
Built during the 1890s and later expanded during the Taish? period, the Lake Biwa Canal played an important role in connecting ?tsu and Kyoto, facilitating water and passenger transportation and also providing electrical energy to power Japan's first streetcar railroad services. The city was founded on October 1, 1898.
On March 20, 2006 the town of Shiga, in Shiga District ceased to exist after merging into ?tsu
