Ending hunger is a key goal of the 2030 Agenda for Sustainable Development, adopted in 2015. This goal notwithstanding, the prevalence of severe food insecurity of the world’s population has increased. It is highest in Sub-Saharan Africa, where the seasonality of harvests leads to fluctuations in food insecurity, particularly in the lean season, the time before the harvest is brought in. We posit that addressing seasonal food insecurity requires not only increased food production, as is commonly argued, but also consideration of post-harvest losses during storage. Here we present the results of a randomized control trial on the effects of improved on-farm storage on seasonal food insecurity. Our intervention provided farming households from two districts in Tanzania with hermetic storage bags that can help reduce storage losses. Seasonal food insecurity was measured via multiple rounds of SMS-based surveys. The results show that the intervention reduced the proportion of severely food insecure households by 38% on average in the lean season, and by 20% in the full seasonal cycle. These findings demonstrate that a simple and inexpensive technology could contribute strongly to reducing seasonal food insecurity and improving smallholder farmers’ year-round access to food.