The aim of this study was to analyze the levels of stress, depression, and anxiety among Polish secondary school teachers and their association with selected psychosocial, sociodemographic, and lifestyle factors. A cross-sectional study was conducted among 116 Polish teachers in 2019, using the International Physical Activity Questionnaire—Short Form (IPAQ-SF), the Depression and Anxiety Stress Scale (DASS-21), and author questionnaire including psychosocial factors specific to the respondents’ work environment. The prevalence of moderately to extremely severe levels of depression, anxiety, and symptoms of stress in teachers was 18.1%, 22.4%, and 51.7%, respectively. Among respondents with moderate or high physical activity level (PAL), normal or mild levels of depression (OR = 3.62; CI [1.31–10.03]), anxiety (OR = 2.61; CI [1.01–6.73]), and stress (OR = 2.79; CI [1.16–6.69]) were more common. The level of individual symptoms of mental disorders was higher than indicated by previous Polish reports. Given that teachers’ low PAL is significantly related to their moderately to extremely severe levels of stress, depression, and anxiety, we suggest running leisure-time physical activity (LTPA) enhancement programs and supporting the leadership of school management and the higher-education system in this regard.