[5] Jackson Township is located in northern Dauphin County, in the Ridge and Valley region of the Appalachian Mountains.

It is a long, narrow township, which extends east to west, with its northern border following the crest of Berry Mountain.

The western half of the township is an agricultural area surrounding the valley of Armstrong Creek, while the eastern half is occupied by Berry Mountain and Broad Mountain, with some portions being part of Weiser State Forest.

The unincorporated communities of Fisherville, Enders, and Dietrich are located in the western half of the township.

According to the United States Census Bureau, the township has a total area of 40.1 square miles (103.8 km2), all land.