At least two gunmen were involved in the incident at the Cameo Night Club, the Cincinnati Police Department said.
Several people have life-threatening injuries, assistant police chief Paul Neudigate told news channel WLWT-TV.
"We are in the middle of a very horrific situation that occurred at the nightclub with multiple victims," Mr Neudigate said.
"It's going to be a long night for our homicide units to investigate," he added.
Police Captain Kimberly Williams said the gunmen were still at large and the force did not have good descriptions of them. Witnesses had been reluctant to co-operate, she added.
"At this point it's unclear exactly what instigated the shooting," Capt Williams told reporters. "We believe that there were at least a couple of shooters involved."
The shooting happened at about 01:00 (05:00 GMT) while hundreds of people were in the venue. There was "chaos" in the aftermath, Captain Williams said.
The Cameo Night Club had had problems in the past "but this is by far the worst", she added.
The shooting comes less than a year after gunman Omar Mateen opened fire at a gay nightclub in Orlando, Florida.
Mateen killed killing 49 people in what was the deadliest mass shooting in US history.
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