Williamsdale is a census-designated place (CDP) in St. Clair Township, Butler County, Ohio, United States.

Williamsdale is located in the southern part of St. Clair Township, just north of the village of New Miami.

U.S. Route 127 forms the western edge of the CDP, leading south into New Miami and thence into the city of Hamilton, and north to Eaton.

The unincorporated community of Overpeck is directly to the northeast, along Hamilton Trenton Road.

According to the United States Census Bureau, Williamsdale has a total area of 0.17 square miles (0.45 km2), all land.