Fairfield Township is a township in Lycoming County, Pennsylvania. Fairfield Township's population has grown since the 1890 census when 468 citizens were counted. The census of 2000 counted 2,659 citizens. It is part of the Williamsport, Pennsylvania Metropolitan Statistical Area.

Fairfield Township is bordered by Upper Fairfield Township to the north, Muncy Township to the east and the West Branch Susquehanna River to the south. The western border is bounded by Montoursville and Loyalsock Creek which forms the border with Loyalsock Township to the west.

The exact date of the formation of Fairfield Township is not known, but court records point to sometime between August 1825 and December 1826 from Muncy Township. A petition for the division of Muncy Township was read in court in August 1825. Historical records do not record when the township was formed, but court documents cite Fairfield Township as the residence of a defendant in a lawsuit in a Columbia County court session in 1826.

Fairfield Township has many residential neighborhoods that are contiguous with the neighboring borough of Montoursivlle.