William Penn High School, also known as William Penn or simply Penn, is a four-year comprehensive, coeducational public high school located in unincorporated New Castle County, Delaware, United States.

It is adjacent to the New Castle city limits and has a New Castle postal address.

The school mascot is the Colonial.

In addition to New Castle the school district includes Delaware City, Port Penn, St. Georges, Wilmington Manor, portions of Wilmington, and half of Bear.

The present building was designed by Wilmington architects Wason, Tingle & Brust and completed in 1966.