Background Adolescence is a crucial period in the development of smoking behaviour. To develop efficient prevention programmes for teenagers, it is essential to understand why adolescents start to smoke. The objective of this study was to assess the predictors of smoking onset among Romanian junior high school students aged 13–14. Methods The data were obtained from a two‐wave, 9‐month longitudinal study carried out among 504 junior high school non‐smokers from Cluj‐Napoca, Romania. Questionnaires assessed smoking behaviour, attitudes, social influence, self‐efficacy and intention regarding smoking (motivational variables), as well as different sociodemographic features. Results The results from the logistic regression analysis revealed that baseline lower self‐efficacy in refraining from smoking in several social situations, baseline pressures from peers to smoke and baseline intentions to smoke significantly increased the risk of non‐smokers to become smokers at follow‐up 9 months later. Conclusions These findings underline that reinforcing social self‐efficacy to refuse smoking, resisting peer pressures and maintaining negative intentions regarding smoking are essential ingredients for smoking prevention programmes among Romanian junior high school students.