National disaster agency spokesman Sutopo Purwo Nugroho said a hill in the Nganjuk district of East Java collapsed.
"Rocks, debris and earth fell on to tourists who were bathing at the foot of the waterfall," Nugroho said.
The waterfall is located at the foot of Mount Wilis, a dormant volcano in East Java.
Tourism sites across the country are swamped with holidaymakers during the week-long Eid festivities following the Ramadan fasting month.