The lockdown imposed as a result of the COVID‐19 pandemic has placed unprecedented stress on families. The family is a key system in relation to child development, and when birth families are unable to meet their child's needs, foster carers become an important source of support in their development. The Double ABC‐X Model of family stress and adaptation considers that a family's capacity to cope with stressful situations is influenced by pile‐up of stressors, family resources and perception of the situation. Following this model, the study aimed to determine the impact of the COVID‐19 lockdown on family stress and functioning. The sample comprised 347 Spanish adults (100 with foster families and 247 with non‐foster families) who completed a survey during lockdown. The results showed that certain sociodemographic variables (gender, income, working from home, characteristics of the home and children with special educational needs) were associated with stressors during lockdown, perception of the lockdown, and family adaptation to stress during lockdown. Associations were also found between stressors, family resources, perception of lockdown, and family adaptation in foster and non‐foster families, with the former having a more positive perception of the lockdown experience. The relevance of the results and their implications are discussed.