Objective: The primary aim of this study was to describe and analyse the physical activity and sedentary levels of secondary school teachers in Flanders. A secondary aim was to collect information regarding a possible worksite intervention of special relevance to secondary school teachers. Design: Mixed-methods quantitative and qualitative enquiry. Setting: Secondary school teachers in Flanders. Method: An online survey was conducted consisting of the International Physical Activity Questionnaire, demographic information and teaching aspects. Additionally, four focus groups (n = 27) took place. Results: Respondents reported an average of 137.2 ± 96.8 minutes/day of physical activity and an average sitting time of 4.6 ± 4.2 hours/day. Overall, 66% of Flemish teachers met the physical activity guidelines of at least 150 minutes of moderate to vigorous activity a week. Lower levels of activity were more prevalent among female teachers, teachers working in technical and general education and those teaching theoretical courses. In focus group discussions, two types of barriers were identified influencing teachers' physical activity at school: organisational barriers such as planning and lack of accommodation, and individual barriers such as having other responsibilities. Conclusions: Interventions can be developed to target less active teachers. However, a classic worksite physical activity intervention during working hours is not suitable for most teachers. Promoting leisure-time activity and active commuting are more fitting components of an intervention for this target group. Actions at school should be achievable and unforced, supporting teachers' development of physical literacy.