Purpose: Little is known about the consequences of COVID-19 lockdowns on physical activity (PA), eating behavior, and mental health in post-bariatric surgery (BS) patients. We aimed to analyze the relations between changes in PA during COVID-19 lockdowns and changes in body weight and a comprehensive set of lifestyle and psychological outcomes in patients who have undergone BS. Material and Methods: In April–May 2020 (lockdown#1), we performed an online survey in a cohort of 937 adults who underwent BS and were followed-up at our university medical center for at least one year. We assessed changes in PA, eating behavior, body weight, fatigue, and depression (PHQ-9). In November–December 2020 (lockdown#2), we recorded body weight in 280 patients who had reported decreased PA during lockdown #1. Results: During lockdown #1 (N = 420 patients included, 44% response rate), decreased PA was reported by 67% patients. Compared to those who reported increased or unchanged PA, patients with decreased PA were more likely to report a ≥ 5% weight gain (OR (95% CI): 3.15 (1.46–7.65), increased fatigue (2.08 (1.36–3.23)), a worsening of eating behavior (2.29 (1.47–3.58)), and moderate-to-severe depressive symptoms (4.74 (2.14–11.76)). During lockdown #2 (N = 225 patients, 80% response rate), significant weight gain since before lockdown #1 was reported (+ 2.8 (95% CI: 1.7–3.8) kg, p < 0.001), with 36% patients reporting a ≥ 5% weight gain. Conclusions: PA may counteract detrimental effects of COVID-19 lockdown on post-BS weight trajectories and mental health outcomes. Follow-up measures are needed in this setting to assess the long-term impact of lockdown.