Introduction: Children are particularly vulnerable to the health effects of secondhand smoke (SHS). The objectives of this study are to describe SHS exposure of children younger than 12 years in Spain and to identify potential social inequalities associated with SHS exposure. Material and Methods: A cross-sectional study was conducted in a representative sample of the population younger than 12 years in Spain. A computerassisted telephone interview was conducted with parents or legal guardians in 2016, to assess the children's SHS exposure at home, in the car, at school and at the nursery gates, in public transport, and during leisure time. The socio-demographic variables included were the child's age and sex, the highest educational attainment at home, and occupational social class. Prevalence and 95% confidence intervals were calculated for SHS exposure in each setting and for overall exposure. Results: In all, 71.8% of the children were exposed to SHS: 25.8% were exposed at home, 4.6% in the car, 8.2% in public transport, 31.9% at outdoor nursery or school gates, and 48% during leisure time. The higher the educational attainment at home, the lower the exposure (38.8% for primary school or lower, 28.7% for secondary school and 20.8% university level). The more deprived the social class, the higher the exposure (21.7% class I-II, 23.4% class III-IV and 31.1% class V-VII). SHS exposure in cars and overall exposure also decreased with higher educational achievement. Conclusions: In Spain, a large proportion of children are still exposed to SHS. Furthermore, there are clear social inequalities. To reduce SHS exposure, there is an urgent need for evidence-based interventions with an equity perspective.