Objective Our aim was to investigate if physical activity (PA) in bariatric surgery patients is related to temperament. Methods Preoperative (n = 70) and post‐operative (n = 73) patients were categorized as being physically ‘active’ versus ‘inactive’ on the basis of objective PA monitoring. Assessment included the behavioural inhibition system (BIS)/behavioural activation system (BAS) scales, the effortful control (EC) subscale of the Adult Temperament Questionnaire‐Short Form, a numeric pain rating scale and measures for depressive and eating disorder symptoms. Results ‘Active’ did not differ from ‘inactive’ patients with regard to temperament (BIS, BAS, and EC). Regressions with PA grouping as dependent variable (adjusted for age, gender, body mass index (BMI), depressive or eating disorder symptoms, or pain intensity) indicated an association between lower BMI and more PA in the preoperative and the post‐operative group. In the post‐operative group, in addition to lower BMI, also lower age and higher BIS reactivity contributed to more PA. Furthermore, there was a significant interaction between BMI and BIS suggesting that low BMI was only associated with more PA in post‐operative patients with high BIS. Discussion The results indicate that temperament per se does not contribute to the level of PA in bariatric surgery patients. However, in post‐operative patients, lower BMI was associated with a higher likelihood of being physically active particularly in patients with anxious temperament. These preliminary findings need further investigation within longitudinal studies. Copyright © 2017 John Wiley & Sons, Ltd and Eating Disorders Association.