Authorities raised the alert level for the area, banning people from approaching within 1.25 miles of the volcano.
The region is sparsely populated, and with no homes in the immediate are there were no reports of injuries or damage.
The Japan Meteorological Agency said it was unclear whether there was any volcanic flow from the mountain.
"This could happen a few times," Yasuaki Sudo, a researcher at the Aso Volcano Museum, told NHK. "The most important thing is to stay out of the danger zone."
There are more than 100 volcanoes in Japan, which sits on the "ring of fire," which circles the Pacific Ocean. Mount Aso had a smaller eruption in August and eruptions in 2014 that disrupted tourism in the area.
The Associated Press contributed to this report.