Marietta is a borough in Lancaster County, Pennsylvania, United States.

Pennsylvania Route 441 passes just north of the borough and forms part of its northeastern border; the highway leads southeast 3.5 miles (5.6 km) to Columbia and northwest (upriver) 15 miles (24 km) to Middletown.

In 1727, James Anderson made a lottery that laid out part of present-day Marietta.

Further development by Jacob Grosh, John Myers, John Peadan, James Mehaffey, Benjamin Long, and James Huffy laid out the rest of present-day Marietta.

Thereafter, industries such as timber and iron smelting provided many economic opportunities in the borough.

Primary and secondary education in the borough of Marietta is provided by the Donegal School District.

Students kindergarten through second grade attend the Donegal Primary School in Mount Joy.