is a town located in Nagano Prefecture, Japan. , the town had an estimated population of 4,111 in 1751 households, and a population density of 19 persons per km<sup>2</sup>. The total area of the town was . Nagiso is listed as one of The Most Beautiful Villages in Japan.
Nagiso is located in the Kiso Valley of southwestern Nagano Prefecture, bordered by Gifu Prefecture to the west. The Kiso River flows through the center of the town.
The town has a climate characterized by characterized by hot and humid summers, and cold winters (Köppen climate classification Cfa). The average annual temperature in Nagiso is . The average annual rainfall is with July as the wettest month. The temperatures are highest on average in August, at around , and lowest in January, at around .
The area of present-day Nagiso was part of ancient Shinano Province. During the Edo period, the area developed as series of post stations on the Nakasendà  highway connecting Edo with Kyoto. The villages of Yomikaki, Tsumagoi and Tadachi merged to form the town of Nagiso on May 1, 1968.
Per Japanese census data, the population of Nagiso has remained declined rapidly over the past 60 years.
Nagiso has one public elementary schools and one public middle school operated by the town government, and one high school operated the Nagano Prefectural Board of Education.