Background-- Most metabolic and bariatric surgery (MBS) patients perform too little moderate-to-vigorous intensity physical activity (MVPA) and too much sedentary time (ST). Identifying factors that influence MVPA and ST in MBS patients is necessary to inform the development of interventions to target these behaviors. Research has focused on individual-level factors and neglected those related to the physical environment (e.g., weather and pollution). These factors may be especially important considering rapid climate change and emerging data that suggest adverse effects of weather and pollution on physical activity are more severe in people with obesity. Objectives--To examine the associations of weather (maximal, average and Wet Bulb Globe Temperatures), and air pollution indices (air quality index [AQI]) with daily physical activity (PA) of both light (LPA) and MVPA and ST before and after MBS. Methods--Participants (n=77) wore an accelerometer at pre- and 3, 6, and 12-months post-MBS to assess LPA/MVPA/ST (min/d). These data were combined with participants local (Boston, MA or Providence, RI, USA) daily weather and AQI data (extracted from federal weather and environmental websites). Results--Multilevel generalized additive models showed inverted U-shaped associations between weather indices and MVPA (R2≥.63, p<.001), with a marked reduction in MVPA for daily maximal temperatures ≥20C. Sensitivity analysis showed a less marked decrease of MVPA (min/d) during higher temperatures after versus before MBS. Both MVPA before and after MBS (R2=0.64, p<.001) and ST before MBS (R2=0.395; p<.05) were negatively impacted by higher AQI levels. Discussion--This study is the first to show that weather and air pollution indices are related to variability in activity behaviors, particularly MVPA, during pre- and post-MBS. Weather/environmental conditions should be considered in MVPA prescription/strategies for MBS patients, especially in the context of climate change.