Fibromyalgia (FM) is a chronic pain condition that negatively affects patients’ quality of life and functionality. Recent studies indicate that emotional disclosure can positively modulate pain perception, cognitive performance and other FM symptoms. This study evaluated the effects of Emotional Writing Therapy (EWT) on emotional variables, clinical pain, cognitive performance (attention, processing speed, mental flexibility and motor function) and functional outcomes (fatigue, quality of life, sleep and overall disease impact) in patients with FM. Two groups were randomly assigned: a disclosure group (30 patients who wrote about stressful events) and a neutral group (30 patients who wrote about time management). EWT lasted 4 days. Psychological and functional assessments were conducted, as well as the Trail Making Test (TMT) and an emotional task (Picture Frame, PF), before and after the intervention. Daily variables, such as mood, anxiety, affect, pain, perceived stress, fatigue and psychological well-being, were measured using Visual Analogue Scales (VASs). At the end of the intervention, a satisfaction questionnaire was administered, and follow-ups were conducted at 1, 3 and 6 months. Both groups showed comparable pre–post improvements in emotional factors, pain perception, and quality of life. However, a significant group × time interaction indicated that only the emotional writing group achieved greater reductions in anxiety, mood disturbance, and stress, along with increases in positive affect and psychological well-being from the second day onward. Overall, emotional writing yields specific emotional benefits beyond those of time-management writing, although both approaches are beneficial. Thus, the choice of intervention should be tailored to the patient’s clinical profile.