The city was founded on October 1, 1898. As of October 1, 2010, the city has an estimated population of 338,629 with an average age of 40.7 years (15.42% of whom are under 15 and 16.81% are over 65) and a population density of 905.28 persons per km2. The total area is 374.06 km2.
In the years 667 to 672, the Ōmi Ōtsu Palace was founded by Emperor Tenji. The Jinshin War devastated Otsu, but a new capital Heian-kyō (now Kyoto) was established the immediate neighborhood in 794, and Otsu was revived as important traffic point and a satellite town of the capital. In the 12th century, the Battle of Awazu occurred in southern Ōtsu, during which Kiso Yoshinaka was killed.
On 11 May 1891, the Ōtsu incident, a failed assassination attempt on Tsarevich Nicholas Alexandrovich of Russia (later Tsar Nicholas II), occurred.
Lake Biwa, the largest lake in Japan, is in the center of the prefecture and is also a site of significant relevance to the Japanese tourism industry. Scenic spots of Otsu have been known as The Eight Views of Omi and attracted literary men such as Matsuo Bashō. Sightseeing tours usually take up to several hours and tourists are given an opportunity to swim and take long strolls around the region. Lake Biwa is also a famous beach spot in the summer. The city has an annual road race named after the lake: the Lake Biwa Marathon.
Built during the 1890s and later expanded during the Taishō period, the Lake Biwa Canal played an important role in connecting Ōtsu and the nearby city of Kyoto, facilitating water and passenger transportation and also providing electrical energy to power Japan's first streetcar railroad services.
On March 20, 2006 the town of Shiga, in Shiga District ceased to exist after merging into Ōtsu.