Interior architecture is the design of a building or shelter from inside out, or the design of a new interior for a type of home that can be fixed. It can refer to the initial design and plan used for a building's interior, to that interior's later redesign made to accommodate a changed purpose, or to the significant revision of an original design for the adaptive reuse of the shell of the building concerned. The latter is often part of sustainable architecture practices, whereby resources are conserved by "recycling" a structure through adaptive redesign.

Generally referred to as the spatial art of environmental design, interior architecture also refers to the process by which the interiors of buildings are designed to address all aspects of the human use of their structural spaces. Put simply, interior architecture is the design of an interior in architectural terms.