William Allen High School, often referred to as Allen High School or simply Allen, is one of two large, urban public high schools of the Allentown School District in Allentown, Pennsylvania. The school provides public education for grades 9 through 12. William Allen High School is located at 106 N. 17th Street in Allentown. It serves students from center city and the city's westside. The city's other public high school, Dieruff High School, serves students from Allentown's eastern and southern sections. Until Dieruff's opening in 1959, William Allen High School was known as Allentown High School.

As of the 2023–24 school year, the school had an enrollment of 2,984 students, according to National Center for Education Statistics data.

High school students may choose to attend Lehigh Career and Technical Institute for vocational training in the trades. The Carbon-Lehigh Intermediate Unit, IU21, provides the Allentown School District with a wide variety of services like specialized education for disabled students and hearing, speech and visual disability services and professional development for staff and faculty. In 2015, the school district opened Building 21 Allentown as an experimental high school which focuses on building passion among students through career exploration.

William Allen High School is named in honor of William Allen, who founded Allentown in 1762, and served as mayor of Philadelphia and chief justice of the Province of Pennsylvania during the colonial era.