The aim of the study was to examine whether weight reduction can result in improvement of fibromyalgia impact questionnaire (FIQ) in the patients with fibromyalgia syndrome (FMS). This study was a randomized controlled trial. Obese patients with fibromyalgia were randomly assigned to 6-month dietary weight loss (n = 41) and no weight loss (n = 42) groups. Patients were assessed at baseline and at 6 months. The primary outcome measure was FIQ. Secondary measures included the tender point (TP) examination, Beck Depression Inventory-II, and Pittsburg Sleep Quality Index. Compared to the control group, patients who underwent weight reduction obtained significantly better FIQ (p = 0.007), lower mean TP count (p = 0.015), and lower mean TP pain rating in the lower body (p < 0.001). Patients who lost weight had less depression and better sleep quality than the controls. Patients who lost weight had significantly lower interleukin 6 and C-reactive protein levels than those in the control group (p = 0.034 and p = 0.007, respectively). Weight loss in obese patients with FMS leads to significant improvement in the quality of life as shown by the decrease in the FIQ score. Depression, sleep quality, and tender point count are also significantly improved by weight loss in obese patients with fibromyalgia. Our results suggest that weight reduction should be a part of fibromyalgia treatment.