Objective: To advance our understanding of poststroke fatigue by investigating its momentary and time-lagged relationship with daily activities. Design: Longitudinal observational study using the experience sampling method (ESM). Setting: Outpatient rehabilitation care. Participants: Thirty individuals with stroke (N=30). Interventions: Not applicable. Main Outcome Measures: ESM is a structured diary method that allows assessing real-time symptoms, behavior, and environment characteristics in the flow of daily life, thereby capturing moment-to-moment variations in fatigue and related factors. Using a mobile application, individuals with stroke were followed during 6 consecutive days, and were prompted at 10 random moments daily to fill in a digital questionnaire about their momentary fatigue and current activity: type of activity, perceived effort and enjoyment, and physical activity levels. Results: Based on all completed digital questionnaires (N=1013), multilevel regression analyses showed that fatigue was significantly associated with type of activity and that fatigue was higher when participants had engaged in physical activity. Fatigue was also higher during activities perceived as more effortful and during less enjoyable activities. Time-lagged analyses showed that fatigue was also predicted by physical activity and perceived effort earlier during the day. Importantly, the relationship between these daily activity characteristics and fatigue differed substantially across individuals. Conclusions: This study illustrates the need for ESM to design personalized rehabilitation programs and to capture fatigue and other patient-reported outcomes in daily life.