The shooting happened outside the Ned Peppers bar in the Oregon District, a busy neighborhood full of bars and restaurants in the historical center of Dayton.
Police said that "thousands of people" would be in the area on a summer Saturday night.
Dayton mayor Nan Whaley suggested the death toll would have been much higher if the police weren't in the district at the time.
“The officers were there less than a minute from the beginning of the shooting. The shooter was able to kill nine people and injure 26 in less than a minute," mayor Nan Whaley said during a news conference.
"If we did not have police in the Oregon District ... what we could have had in this city ...,” she added.