As of 1 October 2016, the town had an estimated population of 20,323 in 9897 households, and a population density of 130 persons per km². The total area of the town is 156.03 square kilometres (60.24 sq mi). Karuizawa is a popular summer resort area for the residents of Tokyo.
Karuizawa is located in eastern Nagano Prefecture, bordered by Gunma Prefecture to the north, east and south. The town is located on an elevated plain at the foot of Mount Asama, one of Japan’s most active volcanoes. The mountain is classed as a Category A active volcano. A small eruption was detected in June 2015, a more significant eruption spewing hot rocks and a plume of ash occurred in February 2015. Mt. Asama’s most destructive eruption in recent recorded history took place in 1783, when over 1,000 were killed. The volcano is actively monitored by scientists and climbing close to the summit is prohibited.