Fishways are often presented as a parsimonious way of assisting water infrastructure development in developing countries while preserving some ecosystem values. More specifically, fishways help preserve migratory freshwater fish species, which are often a main source of protein and other nutrients for many rural communities. To date, much of the literature regarding fishway development focuses on increasing understanding of the interaction between the fish populations and engineering interventions. Accordingly, there is limited analysis that considers fishway construction relative to the specific benefits and costs that subsequently accrue to humans. In this study we outline the development of a decision support tool designed to assist officials in the Lower Mekong evaluate the merits of constructing fishways to address barriers to fish passage. Arguably, the tool can also be modified to assess the avoided cost of building additional barriers that constrain fish migration. The Lower Mekong Fishway Support Tool (LMFST) is underpinned by economic notions and populated with input from experts involved in fishway construction. The LMFST is intended to provide indicative monetary estimates and does not replace comprehensive engineering and social analysis; rather, it guides users in assessing those locations and structures that are more likely to deliver a positive outcome for local communities. The LMFST is buttressed in benefit–cost analysis and encourages decision making to be more transparent and this is one of the major contributions of this work.