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.

William Allen High School was established in 1858 by R. W. McAlpine, who took a group of 14 older students to the Garber-Horne Building, formerly the home of the North American Homeopathic School of Healing Arts at South Penn Street, near the present Allentown School District administration building. This was the beginning of what was then called Allentown High School, a name it retained until 1858. Primary and secondary education originally were both taught in the school. The first class was fourteen pupils, equally divided by boys and girls.

In 1859, a separate high school was established by a vote of 6 to 2, one for boys and one for girls. The second school was located at the Presbyterian Sunday School. Augustus Armagnac was named teacher for the male students, and Hannah L. Romig for the female students. The first commencement was held in May 1869 in the Presbyterian Church on N. 5th Street and included three students.

From 1859 to 1894, secondary school classes were held separately for boys and girls. The classes for boys were held in the Leh's store building on Hamilton Street, and the girl's classes were held in the Sunday School rooms of the First Presbyterian Church on N. 5th Street. Classes were transferred to the Fourth Ward Wolf Building from 1880 to 1894. In 1895, they were moved again to a new building built exclusively as a high school at the site of the "Old Central School" at Lumber and Turner streets, which became overcrowded with students.

In 1913, five rooms of the Herbst School Building were used for the freshman class of 1918.