Rice-aquaculture is promoted in many countries as a system that could simultaneously improve land and water productivity and household diets and nutrition. However, studies evaluating the effects of rice-aquaculture adoption on household diets do not yet exist. Here, we address this research gap, using data from a survey of 720 households in rural Bangladesh and different statistical techniques to control for possible selection bias. Contrary to expectations, our data suggest that adopting rice-aquaculture is associated with a decrease in household dietary quality, especially during the agricultural lean season. Households with young household heads, low education levels, and small landholdings are over-proportionally affected. We also analyze possible mechanisms of these unexpected negative diet effects. Households adopting rice-aquaculture spend much more time on farming, leaving less time for cooking, other domestic tasks, and certain off-farm activities. Adopters have lower crop and livestock production diversity, lower income from forest extraction activities, and higher debts than non-adopters. Our findings suggest that policies to promote the adoption of riceaquaculture should consider the broader effects on household livelihoods and provide sufficient support in order to avoid undesirable social outcomes.