Aso is a city located in Kumamoto Prefecture, Japan. The modern city of Aso was established on February 11, 2005, from the merger of the fo…Aso is a city located in Kumamoto Prefecture, Japan. The modern city of Aso was established on February 11, 2005, from the merger of the former town of Aso, absorbing the town of Ichinomiya, and the village of Namino.