Background Insufficient weight loss and weight regain is seen in 20–30% of the post-bariatric population. More knowledge about the effect of physical activity and eating style on weight change after Roux-en-Y gastric bypass is essential since behaviour can be modified and thereby results improved. The goal of this study is to determine the relationship between weight change, self-reported physical activity and eating style. Methods Weight, physical activity (PA) and eating style (ES) were assessed before surgery and 15, 24, 36 and 48 months after surgery. A linear mixed model was performed to assess the association between the change in PA and ES and percentage total weight loss (% TWL). Results There were 4569 patients included. Preoperative PA and ES were not related to weight change. Change in PA was positively associated with % TWL at 15, 36 and 48 months follow-up. Change in emotional eating was negatively related to % TWL at all follow-up moments. Change in external eating was only negatively related to weight loss at 24 months follow-up. Change in restrained eating was negatively associated with weight loss up to 36 months follow-up. More restrained eating at 36 months follow-up was related to higher weight regain, and more emotional eating at 48 months to 48-month weight regain. Conclusion Preoperative self-reported PA and ES did not predict weight change after RYGB. Being are more physically active and showing less emotional and restrained eating was related to a higher weight loss. Emotional and restrained eating were related to higher weight regain.