Sexual identity disparities in smoking behaviours are well established; however, there is limited research on whether these disparities have diminished as the social and political landscape has changed for lesbian, gay and bisexual people. Thus, we examined changes in prevalence and sexual identity disparities in three smoking behaviours among Canadian adolescents from 1998 to 2013. Data are from the provincially representative British Columbia Adolescent Health Survey (N = 99 373). Using sex-stratified, age-adjusted logistic regression models, we estimated: (i) trends in lifetime cigarette use, early onset, and past 30-day use for heterosexual and three subgroups of sexual minority (i.e. mostly heterosexual, bisexual and gay/lesbian) youth; (ii) sexual identity disparities in these cigarette-related behaviours within each survey year (1998, 2003, 2008, 2013); and (iii) whether the size of the disparity has changed from 1998 to 2013. Smoking has declined for all youth from 1998 to 2013, although less consistently for sexual minority youth. Within-year disparity estimates indicated elevated prevalence of cigarette use for sexual minority compared to heterosexual youth, particularly among females. Trends in sexual identity and smoking behaviours indicated that the degree of differences between heterosexual and sexual minority youth have remained stable or, in some cases, widened. Heterosexual and sexual minority youth differences widened for early onset among sexual minority boys and lifetime and past 30-day use for sexual minority girls. Efforts to prevent smoking behaviours among youth should continue. Tailored preventive strategies for sexual minority youth might help address existing disparities.