Tobacco use often starts in adolescence and is a leading cause of premature mortality. Two previous rounds of the global youth tobacco survey (GYTS) found that a significant proportion of Thai youth currently smoke. We conducted the third round of GYTS in Thailand in 2015 to monitor trends in tobacco use. We selected 31 public and private secondary schools using random sampling based on probability proportional to school enrolment. In each school, we selected 1-3 classes (Grades 7-9) by random sampling. All students in these classes from 30 schools (one school declined) completed a self-administered standard questionnaire in the Thai language. The association between tobacco use and independent variables was examined using univariate and multivariable logistic regression analysis. Of 1876 students, 1721 were aged 13-15 years. Overall, 15% of students currently used tobacco; boys 21.8% and girls 8.1%. The prevalence of current cigarette smoking was 11.3%; 3.3% students currently used electronic cigarettes. Exposure to secondhand smoke (SHS) at home, school, and in enclosed public places was reported by 33.8%, 47.9%, and 38.6%, respectively. Among current smokers, 44% were not stopped from purchasing cigarettes despite being underage. Higher tobacco use was associated with being older, male, exposed to SHS, in possession of an object with a tobacco logo, and being offered a free tobacco product by a tobacco company. Tobacco and cigarette use among Thai students remains high. Underage current smokers have easy access to cigarettes. Urgent steps are needed to curb the access of youth to tobacco.