根西县（Guernsey County, Ohio）是美国俄亥俄州东南部的一个县。面积1,368平方公里。根据美国2000年人口普查，共有人口40,792。县治剑桥 (Cambridge)。

成立于1817年1月31日。本县以殖民者的故乡、英吉利海峡上的根西岛命名。

Guernsey County is a county located in the U.S. state of Ohio. As of the 2010 census, the population was 40,087. Its county seat is Cambridge, and it is named for the Isle of Guernsey in the English Channel, from which many of the county's early settlers emigrated.