BackgroundRecently, the US Institute of Medicine has proposed that raising the minimum age for tobacco purchasing/sales to 21 years would likely lead to reductions in smoking behavior among young people. Surprisingly few studies, however, have assessed the potential impacts of minimum-age tobacco restrictions on youth smoking.ObjectiveTo estimate the impacts of Canadian minimum age for tobacco sales (MATS) laws on youth smoking behaviour.DesignA regression-discontinuity design, using seven merged cycles of the Canadian Community Health Survey, 2000–2014.ParticipantsSurvey respondents aged 14–22 years (n=98 320).ExposureCurrent Canadian MATS laws are 18 years in Alberta, Saskatchewan, Manitoba, Quebec, the Yukon and Northwest Territories, and 19 years of age in the rest of the country.Main outcomesCurrent, occasional and daily smoking status; smoking frequency and intensity; and average monthly cigarette consumption.ResultsIn comparison to age groups slightly younger than the MATS, those just older had significant and abrupt increases immediately after the MATS in the prevalence of current smokers (absolute increase: 2.71%; 95% CI 0.70% to 4.80%; P=0.009) and daily smokers (absolute increase: 2.43%; 95% CI 0.74% to 4.12%; P=0.005). Average past-month cigarette consumption within age groups increased immediately following the MATS by 18% (95% CI 3% to 39%; P=0.02). There was no evidence of significant increases in smoking intensity for daily or occasional smokers after release from MATS restrictions.ConclusionThe study provides relevant evidence supporting the effectiveness of Canadian MATS laws for limiting smoking among tobacco-restricted youth.