Cadiz is the county seat of Harrison County, Ohio. Established in 1803, residents named the town after the Spanish city Cadiz. The town was laid out at the junction of two roads. One road came from Pittsburgh, Pennsylvania, and the other came from Washington, Pennsylvania. Situated at this crossroads, Cadiz grew quickly. In 1840, 1,028 residents lived in Cadiz. By 1846, four churches and twenty-one stores were in the community. Cadiz residents worked at manufacturing goods or processing the crops of farmers in the surrounding countryside.
Between 1840 and 1880, Cadiz almost doubled in size. In 1880 the town had three newspaper offices, two churches, and three banks. Several manufacturing businesses made products primarily for farmers. In 2000, Cadiz had a population 3,308 people. Approximately twenty percent of Harrison County residents lived in the town.