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 is a member of the Colonial School District, and is the largest high school in the state of Delaware. 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 school was established in 1921. The present building was designed by Wilmington architects Wason, Tingle & Brust and completed in 1966.

This article's list of alumni may not follow Wikipedia's verifiability policy. Please improve this article by removing names that do not have independent reliable sources showing they merit inclusion in this article AND are alumni, or by incorporating the relevant publications into the body of the article through appropriate citations. (November 2022)

• Cliff Brumbaugh, baseball player
• Dave May, professional baseball player
• Brett Oberholtzer, baseball player
• Brian Oliver, professional basketball player
• Jeff Otah, professional football player, Carolina Panthers
• Kyle Carter, professional football player, Minnesota Vikings
• Devin Smith, professional basketball player for Maccabi Tel Aviv
• Richard B. Weldon, Jr., former Frederick County (MD) Commissioner and member of the Maryland House of Delegates
• Jheanelle Wilkins, politician and member of the Maryland House of Delegates

• List of high schools in Delaware
• Colonial School District (Delaware)
• William Penn

• Official website

39°40′07″N 75°35′04″W / 39.6687°N 75.5845°W / 39.6687; -75.5845