In the current climate change emergency, it is imperative to explore avenues for upcycling agricultural waste in architecture as a mean of recovering resources while retaining them within the local built environment. Opuntia Ficus-indica (OFI) is a species of cactus that is ubiquitous in the Mediterranean basin, cultivated in many areas for its fruit crops. It has favourable growing conditions because of its excellent water retention capabilities and can also be used for soil erosion prevention. However, in some areas of Sicily it can become a weed and cause much waste due to its large branches (Cladodes) being disposed of in landfills, as well as the waste from the pruning of crop plants. In this research, the feasibility of upcycling cladodes from OFI plants was explored, by utilising them as natural fibre reinforcement in composite layers. Different variables (such as age of the plant and post-harvesting treatments) were analysed to understand how they might affect the final mechanical properties of the OFI cladodes as reinforcement fibres. The mechanical properties of the reinforcement were investigated in terms of tensile and flexural properties, and subsequently its potentiality to be used as a fibrous reinforcement into biocompatible composites was investigated. The bio composites reinforced by OFI cladodes can be used to design building elements for traditional buildings in the plants’ cultivation area, with the suggested application being external roof tiles. The potential of OFI fibres of this research offers an excellent example of the valorisation of indigenous natural resources and agricultural waste to trigger circular processes involving the built environment, the landscape as well as the social and economic system in a Mediterranean setting. Utilising abundant, familiar natural plants in retrofitting culturally significant buildings can signify a strong connection between preserving both our natural and built environment.