Cassella is an unincorporated community in Marion Township, Mercer County, Ohio, United States. Located at 40°24′22″N 84°33′8″W / 40.40611°N 84.55222°W (40.4061581, -84.5521770), it lies at an elevation of 955 feet (291 m). Situated at the intersection of State Route 119 and Cassella-Montezuma Road in western Marion Township, it lies in the headwaters of Beaver Creek to the south of the city of Celina, the county seat of Mercer County. Other nearby communities include Maria Stein, 3 miles (4.8 km) to the east, Carthagena, 2 miles (3.2 km) to the north, and St. Henry, 4 miles (6.4 km) to the west.