A three-decker, triple-decker triplex or stacked triplex, in the United States, is a three-story (triplex) apartment building. These buildings are typically of light-framed, wood construction, where each floor usually consists of a single apartment, and frequently, originally, extended families lived in two, or all three floors. Both stand-alone and semi-detached versions are common.

During the late 19th and early 20th centuries, tens of thousands of three-deckers were constructed, mostly in New England, as an economical means of housing the thousands of newly arrived immigrant workers who filled the factories of the area. The economics of the three-decker are simple: the cost of the land, basement and roof are spread among three or six apartments, which typically have identical floor plans. The three-decker apartment house was seen as an alternative to the row-housing built in other cities of Northeastern United States during this period, such as in New York City, Boston, Philadelphia, Baltimore, and Washington, D.C.