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Patients with chronic obstructive pulmonary disease (COPD) accumulate low levels of physical activity. How environmental factors affect their physical activity in the short-term is uncertain. Aim: to assess the short-term effects of air pollution and weather on physical activity levels in COPD patients. This multi-center panel study assessed 408 COPD patients from Catalonia (Spain). Daily physical activity (i.e., steps, time in moderate-to-vigorous physical activity (MVPA), locomotion intensity, and sedentary time) was recorded in two 7-day periods, one year apart, using the Dynaport MoveMonitor. Air pollution (nitrogen dioxide (NO 2), particulate matter below 10 μm (PM 10) and a marker of black carbon (absorbance of PM 2.5 : PM 2.5ABS), and weather (average and maximum temperature, and rainfall) were estimated the same day (lag zero) and up to 5 days prior to each assessment (lags 1–5). Mixed-effect distributed lag linear regression models were adjusted for age, sex, weekday, public holidays, greenness, season, and social class, with patient and city as random effects. Patients (85% male) were on average (mean ± SD) 68 ± 9 years old with a post-bronchodilator forced expiratory volume in 1 s (FEV 1) of 57 ± 18% predicted. Higher NO 2 , PM 10 and PM 2.5ABS levels at lag four were associated with fewer steps, less time in MVPA, reduced locomotion intensity, and longer sedentary time (e.g., coefficient (95% CI) of −60 (−105, −15) steps per 10 μg/m3 increase in NO 2). Higher average and maximum temperatures at lag zero were related to more steps and time in MVPA, and less sedentary time (e.g., +85 (15, 154) steps per degree Celsius). Higher rainfall at lag zero was related to fewer steps and more sedentary time. Air pollution affects the amount and intensity of physical activity performed on the following days in COPD patients, whereas weather affects the amount of physical activity performed on the same day. • Higher air pollution relates to low physical activity in COPD up to four days later. • High temperature relates to high physical activity in COPD only on the same day. • Greater rainfall relates to low physical activity in COPD only on the same day. • Urban and transport planning policy should consider people with chronic conditions. • Research on physical activity in COPD should collect data on environmental factors. [ABSTRACT FROM AUTHOR]