<strong>Background:</strong> Post–COVID-19 fatigue affects millions worldwide; yet, evidence-based management strategies remain limited. Activity pacing, regulating activity to match available energy and minimize symptom exacerbation, may support symptom management, although optimal pacing approaches remain unclear. <strong>Objective:</strong> This study aimed to explore associations between activity pacing strategies delivered through a mobile app and daily fatigue levels in individuals with post–COVID-19 fatigue. <strong>Methods:</strong> In this exploratory observational study, 19 adults with post–COVID-19 fatigue used wearable devices (Fitbit Inspire 3) for objective activity monitoring and a mobile app (FatigueSense) to self-report daily symptoms (fatigue and energy levels) and optionally select activity pacing goals (light, balanced, or active) over a period of 3 months. We examined associations between pacing strategies and symptom outcomes by using mixed-effects linear models with random intercepts. Same-day outcomes (fatigue and energy reported on the day of goal selection) were analyzed controlling for age and sex. Next-day outcomes (fatigue and energy reported the day following goal selection) were analyzed controlling for age, sex, and prior-day symptoms. <strong>Results:</strong> Across 2182 observation days, participants self-selected pacing goals on 816 (37.4%) days, demonstrating symptom-responsive behavior with higher baseline fatigue on pacing days (mean score 1.74, SD 0.59 vs 1.58, SD 0.54 on nonpacing days on a 0-3 scale where 0=“none” and 3=“severe”; <i>P</i>=.004). On pacing days (584/816, 71.6% with complete data from 18 participants), active pacing was associated with reduced same-day fatigue (β=−0.34, 95% CI −0.49 to −0.19; <i>P</i><.001) and increased same-day energy (β=8.8, 95% CI 4.7-12.9; <i>P</i><.001) compared to light pacing. Balanced pacing also showed significant reductions in fatigue (β=−0.15; <i>P</i>=.008) and increases in energy (β=5.8; <i>P</i><.001) compared to light pacing. Next-day effects were attenuated and nonsignificant (fatigue: β=−0.05, <i>P</i>=.53; energy: β=1.1, <i>P</i>=.61). Individual heterogeneity was substantial, with an intraclass correlation coefficient of 0.32, indicating that 32% of the variance was attributable to between-person differences. Among participants trying multiple strategies, 58.3% (7/12) showed meaningful responses (≥0.3-point fatigue reduction) to structured pacing strategies. <strong>Conclusions:</strong> Structured activity pacing strategies (active and balanced) were associated with improved same-day symptom management in individuals with post–COVID-19 fatigue. However, substantial confounding by indication (self-selection of pacing based on symptom state) and individual heterogeneity limit causal interpretation. These exploratory findings warrant testing in randomized controlled trials to establish efficacy and identify responder characteristics. <strong>Trial Registration:</strong>
