Unhealthy eating behaviour and excess body weight have been related to sedentary behaviour, particularly screen time, in adolescents; however, little is known about their associations with the use of social networking sites (SNS). We investigated the associations between time spent using SNS and unhealthy eating behaviours (including breakfast skipping, consumption of sugar-sweetened beverages (SSB) and energy drinks) and body weight in adolescents. Data on 9858 students (mean age: 15·2 (sd 1·9) years) in grades 7 through 12 were derived from the 2013 cycle of the Ontario Student Drug Use and Health Survey – a cross-sectional school-based survey of middle and high school students. The majority (81·5 %) of students reported daily use of SNS and an additional 10·7 % reported using them on an irregular basis. Multivariate logistic regression analyses revealed that the use of SNS was associated with increased odds of skipping breakfast (P
trend<0·01) and consuming SSB (P
trend<0·01) and energy drinks (P
trend<0·01) in a dose–response manner with adjustments for age, sex, ethnicity, socio-economic status, tobacco, alcohol and cannabis use as well as BMI. However, there was no evidence of a significant association between use of SNS and BMI before or after adjusting for all the covariates and unhealthy eating behaviours. In conclusion, our results suggest associations between the use of SNS and unhealthy eating behaviours among youth. Given the popularity of SNS, more efforts are needed to better understand the impact of social networks on eating behaviours and risk of excess weight.