Objective To investigate the relationship between physical activity (PA) and cognitive performance in extreme obesity. Methods Seventy-one bariatric surgery candidates (77.5% women) with a mean body mass index (BMI) of 46.9kg/m2 (SD =6.0) and a mean age of 41.4 (SD =11.9) years completed SenseWear Pro2 activity monitoring for seven days. Cognitive functioning was assessed by a computerized test battery including tasks of executive function (Iowa Gambling Task), visuospatial short-term memory (Corsi Block Tapping Test) and verbal short-term memory (Auditory-Verbal Learning Test). Questionnaires assessing eating disturbances and depressive symptoms were administered. Somatic comorbidities were assessed by medical chart review. Results The level of PA was low with mean steps per day within wear time being 7140 (SD =3422). Most patients were categorized as sedentary (31.0%) or low active (26.8%). No significant association between PA estimates and cognitive performance was found. Lower PA was modestly correlated with higher BMI but not with age, somatic comorbidity or depressive symptoms. Moderated regression analyses suggested a significant interaction effect between depression and PA in predicting performance on the Corsi Block Tapping Test. Patients with (29.6%) and without (70.4%) regular binge eating did not differ with respect to PA or cognitive function. Conclusion The findings indicate no association between daily PA and cognitive performance in morbidly obese patients. Future studies should explore the relationship between the variables with regard to dose–response-questions, a broader BMI range and with respect to potential changes after substantial weight loss due to bariatric surgery.