Many plant species, including Barrel Cacti, have developed long-term evolutionary adaptable traits to survive in extreme climates. The most important trait of cacti in extreme hot dry climates is to reduce water evaporation and increase water storage. The exterior skin of a cactus plays an important role in preventing water evaporation through heat transmission. On the other hand, there have been many passive design strategies applied to the space planning and building design of architecture in hot dry climates. The goal of these passive design strategies is to regulate the penetration of heat into building spaces while creating a strong ventilation system to help bring cool air inside the building. In this paper, adaptations of the Barrel Cactus' exterior skin, along with architectural passive design strategies for hot dry climates (in this caseYazd, Iran) will be discussed and integrated with one another through the concept of Biomimicry. The goal is to design an exterior building skin that is attuned to the environmental conditions of a hot dry climate, based on the successful applicable behaviors demonstrated in the Barrel Cactus. Key architectural features such as natural ventilation and daylight will be informed by the evolutionary cacti adaptations and passive architectural strategies in the design of the building skin in order to increase the possibility of consistent comfort for users of an office building.