Tobacco smoking remains one of the greatest public health problems facing the UK today. It varies significantly by ethnic group. This study aimed to determine whether ethnic differences in smoking behaviour are related to neighbourhood-level, own-group ethnic density across south and east London. The association between ethnic density and individual smoking behaviour was assessed by multilevel logistic regression using the electronic health records of 688 397 general practitioner-registered patients. Restricted cubic splines were created to explore whether the effect of ethnic density on smoking behaviour was nonlinear. Increasing own-group ethnic density was found to be associated with a significant reduction in the odds of being a current smoker in all ethnic groups, except for Black Caribbean women. The relationship between ethnic density and current smoking was found to be nonlinear, with the strength of association varying significantly by sex and ethnic group. These novel findings point to a complex relationship between culture, neighbourhood-level experience of adversity or social support and smoking behaviour, and will allow us to target smoking cessation services differentially to individuals/groups living in relative ethnic isolation, who do not benefit from the potential cultural/social factors associated with reduced tobacco consumption.