ObjectivesThe study aimed to investigate the prevalence of physical activity (PA) and factors influencing PA behaviours among medical students in Southern Thailand.DesignThe study implemented a mixed methods approach. The sequential design consisted of 2 phases: a survey followed by in-depth interviews.SettingThe study was conducted in the 3 campuses of a medical school in Southern Thailand. The preclinical students (years 1–3) studied general and basic science subjects at Nakhon Si Thammarat campus. The clinical students (years 4–6) received clinical training and hospital attachments at Trang or Phuket campuses. Total number of students was 285, with 46–48 students in each class. The study was conducted from September 2015 to February 2016.ParticipantsParticipants were medical students, 18 years old and above, from 3 campuses without disabilities or medical conditions which limited their ability to perform PA.Outcome measuresThe prevalence of the recommended levels of PA was measured using Global Physical Activity Questionnaire (GPAQ). The association between the demographic data and the recommended PA levels were analysed by univariate and multivariate analysis. In-depth interviews and thematic analysis were completed to explore PA behaviours.ResultsA total of 279 (response rate 97.9%) medical students participated in the study. Approximately half (49.5%) of the participants were physically active. The median total energy use was 540 metabolic equivalent-min/week (range 0–5640). Male and preclinical students were more likely to be physically active (p<0.05). Twenty-four in-depth interviews were conducted. Supportive factors included social support from friends and families. Study-related activities and overtime shift work were barriers.ConclusionsMore than half of the medical students have insufficient PA because of study-related activities and overtime shift work. Future studies should focus on finding ways to improve PA in clinical and female students in Thailand.