Ōtsu serves as the capital city of Shiga Prefecture in Japan, situated on the southern and western shores of Lake Biwa. It occupies the southwestern portion of the prefecture, stretching along the lake's southwest shore in an L-shaped configuration. The city ranges from densely populated alluvium depressions near the water to sparsely populated hilly and mountainous areas to the west and south, including the Hira Mountains and Mount Hiei.

Part of ancient Ōmi Province, Ōtsu has been settled since at least the Yayoi period. It served as an important inland water transportation center on Lake Biwa and was the capital from 667 to 672 under the Ōmi Ōtsu Palace. The city prospered during the Edo period as a major post station on the Tōkaidō and Nakasendō highways. In 1898, it was formally raised to city status after being designated the prefectural capital in 1872.

The city is home to Enryaku-ji, a significant Tendai monastery on Mount Hiei that overlooks Kyoto and forms part of the UNESCO World Heritage Site Historic Monuments of Ancient Kyoto. Other notable landmarks include Ishiyama-dera, founded in 749 with a main hall built 850 years ago, and Gichu-ji Temple, the burial place of haiku poet Matsuo Bashō. The Lake Biwa Canal, constructed in the 1890s, remains a designated Historic Site.

Ōtsu offers numerous museums, including the Shiga Prefectural Lake Biwa Culture Museum and the Museum of Modern Art, Shiga. Historically noted for Ōtsu-e folk drawings and the Ōtsu soroban abacus, the city also produces edible chrysanthemums used in Japanese cuisine. The area attracts visitors through its historical sites, such as the location where Murasaki Shikibu is believed to have written Tale of Genji, and its scenic lakeside environment.

As of 1 October 2021, the city had an estimated population of 343,991. It features a humid subtropical climate with warm summers and cool winters, averaging 13.8 °C annually. Transportation is facilitated by JR West and Keihan Electric Railway lines, with Ōtsu Station as the central hub. The nearest airports are Kansai International Airport and Itami Airport in Osaka.