Background According to the Juvenile Protection Act in Korea, no one is allowed to sell, rent, or distribute tobacco products to adolescents. Furthermore, the Framework Convention on Tobacco Control Article 16 prohibits the sale of tobacco products to minors. In this study, we investigated the trends in and associated factors of the ease of cigarette purchase among Korean adolescents from 2005 to 2016. Methods The analyses were based on the data of the Korea Youth Risk Behavior Web-based Survey. We estimated the trends in the ease of cigarette purchase from 2005 to 2016 and evaluated associated factors. Ease of cigarette purchase was defined as the proportion of adolescents who were able to purchase cigarettes from among those who had tried to purchase cigarettes in the past 30 days. Results The ease of cigarette purchase began decreasing since 2008 (81.3%) compared to 2005 (83.9%). It decreased to 76.5% in 2013 and further decreased to 71.4% in 2016. The ease of cigarette purchase increased for adolescents who were in higher grades, smoked larger amounts of cigarettes per day, lived in metropolitan cities, had problematic drinking in the past year, and had close friends who smoked. It decreased in adolescents who had current smokers among their family members. Conclusions Although the proportion of adolescents who were able to purchase cigarettes significantly decreased starting in 2008, more than 70% of Korean adolescents can still purchase cigarettes. Enforcement of the Juvenile Protection Act must be strengthened in order to prevent cigarette use among adolescents. Electronic supplementary material The online version of this article (10.1186/s12889-018-6151-9) contains supplementary material, which is available to authorized users.