Fatigue is a debilitating symptom of fibromyalgia (FM) that has limited treatment options. Some evidence, however, has linked positive social engagement with reduced within-day fatigue.This study elaborated longitudinal within-day and across-day relations between FM fatigue and social enjoyment.One hundred seventy-six women with FM completed 21-day automated diaries assessing morning and end-of-day fatigue, and both afternoon social enjoyment and stress within two social domains: non-spousal and spousal.In the non-spousal domain, analysis supported a mediational path from lower morning fatigue to higher afternoon social enjoyment, which predicted lower end-of-day fatigue, and subsequently, lower next-morning fatigue. Enjoyment exerted a greater impact on within-day fatigue than did stress. Patterns in the spousal domain were similar, but the mediated path was nonsignificant.Positive social engagement offers relief from FM fatigue that carries over across days and may provide an additional target to enhance the effectiveness of current interventions.