 The Dayton, OH Metropolitan Statistical Area (also known as Greater Dayton), as defined by the United States Census Bureau, is an area consisting of three counties in the Miami Valley region of Ohio and is anchored by the city of Dayton.As of 2000 it is the fourth largest metropolitan area in Ohio and the 61st largest Metropolitan Area by Population in the United States.As of the 2010 census, the MSA had a population of 799,232 based on a change in MSA defining criteria as of 2013, which eliminated Preble County.