Background <p>Fatigue is a highly prevalent and debilitating condition among older adults, significantly associated with adverse health outcomes and functional decline. Despite its clinical importance, the complex interplay between fatigue, its multidimensional influencing factors, and lower limb function remains to be fully elucidated. Guided by the Theory of Unpleasant Symptoms (TOUS), this study aimed to empirically test a conceptual model depicting these relationships.</p> Methods <p>A cross-sectional study was conducted in Grade A tertiary hospitals in China between September 2020 and March 2021. A sample of community-dwelling older adults was recruited and completed a battery of standardized questionnaires assessing bodily pain, psychological status, social support, fatigue, and lower limb function. The hypothesized model was tested using structural equation modeling (SEM).</p> Results <p>The structural equation model demonstrated an excellent fit to the data (χ²/df = 1.321, CFI = 0.983, RMSEA = 0.030). Bodily pain, depressed mood, anxious mood, and social support were found to have significant direct effects on fatigue. In turn, both depressed mood and fatigue exhibited significant direct effects on lower limb function. Furthermore, significant indirect effects on lower limb function, mediated through fatigue, were identified for bodily pain, depressed mood, anxious mood, and social support.</p> Conclusions <p>The findings confirm fatigue as a critical mediator in the pathway from physical, psychological, and social factors to lower limb function in older adults. This underscores the necessity for healthcare professionals to integrate routine fatigue assessment and management into geriatric care. Interventions should adopt a multifaceted approach, concurrently addressing psychological distress, optimizing social support, and managing bodily pain. Targeting fatigue represents a promising strategy for preserving mobility and functional independence in this population.</p>