Although food insecurity configures a public health issue in developing countries going through nutrition transition, there is still lack of evidence on how it is affected by social determinants and its relationship with ultra-processed food (UPF) consumption. Using qualitative methods, we investigated the experience of food (in)security among mothers living in the Brazilian Amazon area, identifying aspects of food insecurity promoting UPF consumption. In-depth interviews were performed with 40 women and inductive content analysis was used. Signs of food insecurity included difficulties in food affordability and irregular access to food. Strategies to deal with lack of food quantity took place during food production (growing foods and raising animals), acquisition (gaining food, shopping incentives and food substitutions) and preparation (creativity in cooking). Not being able to afford staple foods was the main aspect of food insecurity promoting UPF consumption, as fresh foods were substituted by UFP options. Our study contributes to the current literature by presenting explanatory insights about the inconclusive quantitative results on the relationship between food insecurity and UPF consumption. Additionally, it supports the need of policies and interventions focused on promoting sustainable food systems and the regional food culture, which may approach food insecurity through an intersectional perspective.