First reports came in at about 01:00 local time of a shooting outside a bar in the Oregon district of the city.
The Dayton Police Department said the suspect in the shooting is also dead.
According to police, the shooter, who is now dead, used a long gun. A motive for the shooting is not known.
Casualties have been taken to a number of hospitals.
Recall that a mass shooting claimed 20 lives at a Walmart centre in El paso, Texas some 13 hours ago.