The use of life cycle assessment (LCA) allows work to go beyond the traditional scope of urban nature-based solutions (NBS), in which ecosystem services are provided to citizens, to include environmental impacts generated over the entire life cycle of the NBS, i.e., from raw material extraction, through materials processing, production, distribution, and use stages, to end-of-life management. In this work, we explored how LCA has been applied in the context of NBS through a critical analysis of the literature. Systems under review were not restricted to one typology of NBS or another, but were meant to cover a broad range of NBS, from NBS on the ground, water-related NBS, building NBS, to NBS strategies. In total, 130 LCA studies of NBS were analysed according to several criteria derived from the LCA methodology or from specific challenges associated with NBS. Results show that studies were based on different scopes, resulting in the selection of different functional units and system boundaries. Accordingly, we propose an innovative approach based on the ecosystem services (ES) concept to classify and quantify these functional units. We also identify and discuss two recent and promising approaches to solve multifunctionality that could be adapted for LCA of NBS.