How climate risks are understood and perceived by farmers is important because it can influence their management practices. Farmers in Northern Thailand who rear fish in floating cages in rivers can cope with modest fluctuations in water levels, but appear vulnerable to more extreme changes in flow. This study used in-depth interviews and a quantitative survey across many sites to explore how fish farmers perceive climate-related risks and understand climate change. Important climate-related risks – such as floods and droughts – vary by season, year and location, and are modified by water infrastructure. Recent experience of negative impacts increases levels of concern about risks. Risks from droughts were perceived to have significantly worsened. The overall level of awareness of climate change among fish farmers is high, suggesting that future work should focus on building on from how farmers manage risks under the current climate, to then take into account climate change. This study shows that a good understanding of risk perception is likely to be important in improving climate risk management, and thus adaptation to climate change.