Built in 1905 and opened in 1906 as Naugatuck High School, Hillside Intermediate School exemplifies Neo-Classical Revival architecture. Designed by the renowned architectural firm McKim, Mead & White and commissioned by philanthropist John Howard Whittemore, the building was placed on a hill overlooking the Naugatuck Green. Its unique site allowed for entrances on three different levels, a feature recognized in Ripley’s Believe It or Not. Upon completion, the school was hailed as “America’s best equipped and most sumptuous public-school building.”

Whittemore’s vision for the town’s civic improvement led to the construction of Salem School in 1894, which initially served both as a grammar school and a high school. However, by 1900, it became clear that a new high school was needed. Whittemore hired McKim, Mead & White around 1901 to design a new facility. This new school, built on a site northwest of the town center (formerly occupied by Mr. F.F. Shaffer's home), was completed in 1905. The old home was moved, and Hillside Avenue was rerouted to provide street entrances on each of the school’s three floors.

The design of Hillside Intermediate School is inspired by Greek and Roman Classical forms, consistent with the Beaux-Arts training of the architects. Constructed of pressed buff brick and pink Milford granite, the school’s architecture resembles a Grecian temple. Triple flights of granite stairs lead to the south entrance beneath a two-story Ionic pavilion. The school’s east facade, visible from downtown, features a monumental Ionic colonnade and bronze anthemion decorations that were restored following a 1961 fire. The building's complex hipped roof, surrounded by copper trim, and the use of limestone trim further emphasize the classical style.

Designated as part of the Naugatuck Center Historic District on July 30, 1999.