Bariatric surgery patients report significant pre‐ to postoperative increases in physical activity (PA). However, it is unclear whether objective measures would corroborate these changes. The present study compared self‐reported and accelerometer‐based estimates of changes in moderate‐to‐vigorous intensity PA (MVPA) from pre‐ (pre‐op) to 6 months postsurgery (post‐op). Twenty bariatric surgery (65% laparoscopic‐adjustable gastric banding, 35% gastric bypass) patients (46.2 ± 9.8 years, 88% female, pre‐op BMI = 50.8 ± 9.7 kg/m²) wore RT3 accelerometers as an objective measure of MVPA and completed the Paffenbarger Physical Activity Questionnaire (PPAQ) as a subjective measure before and 6 months after bariatric surgery. Time (min/week) spent in MVPA was calculated for the PPAQ and RT3 (≥1‐min and ≥10‐min bouts) at pre‐op and post‐op. Self‐reported MVPA increased fivefold from pre‐op to post‐op (44.6 ± 80.8 to 212.3 ± 212.4 min/week; P < 0.005). By contrast, the RT3 showed nonsignificant decreases in MVPA for both ≥1‐min (186.0 ± 169.0 to 151.2 ± 118.3 min/week) and ≥10‐min (41.3 ± 109.3 to 39.8 ± 71.3 min/week) bouts. At pre‐op, the percentage of participants who accumulated ≥150‐min/week of MVPA in bouts ≥10‐min according to the PPAQ and RT3 was identical (10%). However, at post‐op, 55% of participants reported compliance with the recommendation compared to 5% based on RT3 measurement (P = 0.002). Objectively‐measured changes in MVPA from pre‐op to 6 months post‐op appear to be much smaller than self‐reported changes. Further research involving larger samples is needed to confirm these findings and to determine whether self‐report and objective PA measures are differentially associated with surgical weight loss outcomes.