Background Bariatric surgery is currently recognized as being an effective technique for weight loss and the improvement of patients’ postoperative well-being. Objectives The objective of the study was to measure changes in quality of life (QoL) and body mass index (BMI) according to patients’ sex and 2 types of surgical procedures. Setting Longitudinal cohort study using an online platform from a private hospital in West France. Methods Two hundred six patients (38 men and 168 women) undergoing one-anastomosis gastric bypass or sleeve gastrectomy surgery provided online information concerning their QoL and weight both before the operation and then every 3 months over a postoperative period of 24 months. Results BMI clinically decreased on average by 19.6% in the first 3 months and up to 39.2% 24 months after surgery. Slight differences between men and women appeared as from 18 months after the operation, with men experiencing increased BMI between 18 and 24 months, contrary to women whose BMI remained unchanged during the same period. QoL also improved significantly. The average level of women’s quality of life increased between 3 and 15 months after surgery, then decreased between 15 and 24 months. As for men, no change was observed in their improved QoL between 3 and 24 months after the operation. Conclusions This study highlights the importance of optimizing patients’ monitoring, notably around 15 to 18 months after bariatric surgery. This period can be identified as a first “critical” period during which weight regain (especially for men) and diminished self-perceived quality of life (especially for women) appear.