The Boston Block, also known as Aalfs Manufacturing Company, is a historic building located in Sioux City, Iowa, United States.

The city experienced a building boom that began in the late 1880s and continued into the early 1890s.

The Massachusetts Block (no longer extant) on the southwest corner of Fourth and Jackson was six stories tall and had a similar facade as the Boston Block, which is five stories tall on the northeast corner of Fourth and Virginia.

The Boston Block is a Richardsonian Romanesque structure that features rusticated stone veneer with a two-story iron and glass storefront.

This article about a property in Woodbury County, Iowa on the National Register of Historic Places is a stub.