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Physical activity (PA) is essential for physical and mental health, yet remains insufficient across populations. Environmental factors, such as air pollution (AP), are increasingly recognized as dynamic influences on daily PA behavior. This study investigated within-person relationships between AP (particulate matter PM₁₀) and device-based PA (step counts) over 14 days in a sample of 693 adults living in an industrial region in the Czech Republic (9702 daily observations; 47.2% women; Mage = 38.78, SD = 12.28). The sample was recruited using non-probabilistic quota sampling. Using multilevel Bayesian modeling, we found that on days when PM₁₀ was higher than a participant's average across the 14 days, step counts were lower (b = -0.027; 95% CI = [-0.047, -0.006]), while controlling for the effects of temperature, sunshine, and precipitation. Specifically, a 1-SD increase in PM₁₀ was linked to 361 fewer steps. Self-reported AP monitoring tendency did not moderate the within-person relationship between AP and step counts (b = -0.003; 95% CI = [-0.044, 0.038]). Findings highlight the importance of capturing short-term environmental exposures in PA research and underscore the public health relevance of behavioral adaptation under conditions shaped by climate change, including AP. Results may inform individual-level digital interventions focused on adaptive behavior change as well as policy-level urban planning strategies that promote PA under varying environmental conditions.