Background We evaluated if the intragastric balloon (IGB) treatment leads to the increase in physical activity (PA) and whether they are related to cognitive improvements. Methods Fourteen morbidly obese patients (151 ± 24 kg, BMI = 51.8 ± 6.5, 107 ± 26% excess weight, 43.3 ± 10.6 years) underwent 6-day-long, uninterrupted evaluations of PA 1 month before IGB insertion and 1 month after its removal. Results Active energy expenditure and physical activity duration increased by more than 80% (p < 0.001) whereas the number of steps per day by 20% (p = 0.016). There was a pattern of relationships between cognitive improvements and increases in PA (p < 0.05). In particular, working memory improvements correlated with the increase in time spent on light physical activities (r = 0.673, p = 0.004). Conclusion The relationships suggest that an increase in physical activity mediates cognitive improvements in bariatric patients.