Abstract Background In the USA and Canada, cars are a major source of secondhand smoke (SHS) exposure among youth. Little is known about the magnitude of this public health problem in European countries. In this report, we study SHS exposure in vehicles among adolescents across 7 member states of the European Union (EU), with a particular focus on socio-economic characteristics and adolescents’ smoking environment. Methods Data on self-reported SHS exposure in cars during the past seven days was obtained from the 2016/17 cross-sectional SILNE-R study from 14-17-year old adolescents in EU seven countries (N = 10,481). We applied two multivariate logistic regression models with sociodemographic characteristics and mediating smoking-related factors. Results SHS car exposure varied widely across the 7 EU countries: 6% in Finland, 12% in Ireland, 15% in the Netherlands, 19% in Germany, 23% in Portugal, 36% in Belgium and 43% in Italy. Low paternal educational levels were strong correlates of SHS exposure in cars as well as migration background. Other correlates were one’s own smoking status and the relation to the family and peer smoking environment, such as parental smoking, permissive smoking rules at home, and best friends smoking. Conclusions In most of these seven countries, a considerable proportion of youth, particularly those from disadvantaged backgrounds, is exposed to SHS in cars. There is a need to ensure adoption and sustained enforcement of smoke-free car legislation. Given the long-term effectiveness of smoke-free car policies, our finding suggests that such policies can contribute towards reducing smoking inequalities. Key messages We assessed adolescent secondhand smoke (SHS) exposure in cars in 7 EU countries, which varies widely, ranging from 6% in Finland to 43% in Italy. The findings point to a social gradient, environmental factors in SHS car exposure and call for the rapid implementation of smoke-free car legislation.