Abstract. Citizen observatories are a relatively recent form of citizen science, which involve citizens in making environmental observations over a period of time. These observations can help to inform the decision making of local authorities and other stakeholders, creating a platform for two-way interaction between citizens and public agencies. Although citizen observatories can clearly generate many different benefits, they also have an associated cost. There are currently no examples of quantifying the costs and benefits of citizen observatories in the literature, yet this type of analysis is critical if there is to be real uptake of citizen observatories by public agencies more generally. This paper presents and applies a generic methodology for capturing the value of a citizen observatory for flood risk reduction in the Brenta-Bacchiglione catchment using a cost-benefit analysis. The results show that the benefits of implementing a citizen observatory approach outweigh the costs by approximately 2 to 1 and can reduce the annual expected damage to a greater degree than a much more costly structural approach.