St. Joseph County, commonly called St. Joe County by locals, is a county located in the U.S. state of Indiana. As of Census 2010, the population was 266,931, making it the fifth-most populous county in Indiana. Formed in 1830, it was named for the St. Joseph River which flows through it toward Lake Michigan. The county seat is South Bend. St. Joseph County is part of the South BendâMishawaka, IN-MI, Metropolitan Statistical Area.