county, central Pennsylvania, U.S., located midway between the cities of Williamsport and Harrisburg and bordered to the north by Penns Creek Mountain and to the east by the Susquehanna River. Its ridge-and-valley topography also includes Thick, Jacks, and Shade mountains, while Walker Lake and Penns, Middle, and Mahantango (north and west branches) creeks are among the other waterways. Recreational areas include Bald Eagle State Forest.

In October 1755, Indians slaughtered 25 white settlers in the Penns Creek Massacre. Hendrich's Fort, located near present-day Kreamer, protected settlers from Indian raids for more than a century after it was built in 1770. The county was formed in 1855 and named for Simon Snyder, third governor of Pennsylvania. The principal communities are Selinsgrove, Shamokin Dam, and Middleburg, which is the county seat.

Manufacturing (lumber and wood products) and agriculture (field crops and livestock) form the economic base. Area 331 square miles (858 square km). Pop. (2000) 37,546; (2007 est.) 38,113.