Cognitive reappraisal (CR) is one of the most widely studied emotion regulation strategies and is considered central to psychological well-being. Given its protective role in mental health, a growing body of research has examined factors that influence the effectiveness of CR. However, prior studies have been conducted in controlled laboratory settings, typically relying on artificial or experimentally induced stressors, which may limit ecological validity of the findings. The present study aimed to address this gap by investigating CR in the context of real-world stressors. We recruited 100 young adults who used a mobile application, ReApp, to record their daily stressors and practice CR three times per day, over a three-week period. Our findings suggest that the effectiveness of CR is highly context-dependent, and certain aspects may be more relevant and contributing to CR effectiveness than the other. Specifically, higher positive affect and greater stressor severity were associated with more effective CR. By contrast, higher negative affect and lower reappraisal affordance predicted reduced CR effectiveness. We provide the first evidence that CR effectiveness varies across stressor domains, suggesting it may be better suited to some types of real-world stressors (i.e., health-related stressors) than others (i.e., lack of resources). In contrast, neither subjective stress nor the time spent on generating reappraisals influenced CR effectiveness. Being the first outside-laboratory study examining contextual factors contributing to CR effectiveness, these results carry clinical implications, suggesting that CR trainings may be optimized by adjusting practice to specific contextual factors, and warrant the need for further research.