Background Homeopathy has been used in observational and controlled studies to treat patients with fibromyalgia (FM), but none has previously used the remedy Cannabis sativa. Case History A 51-year-old female patient presenting with diffuse pain and sleep disorder was diagnosed with FM using the relevant American College of Rheumatology criteria. She reported having 18 tender points, a pain score (visual analog scale, VAS) of 9.0, and a well-being VAS of 5.0. She was prescribed Cannabis sativa 6 cH, five drops sublingually thrice a day. Results After 2 months, she returned asymptomatic, with 0 tender points, pain VAS of 0, and well-being VAS of 9.0. The Modified Naranjo Criteria for Homeopathy score was equal to +9, suggesting the clinical outcome was causally attributable to the medicine prescribed. Conclusion This case study reveals the positive role of homeopathic treatment in FM. Studies using a randomized controlled design, including pragmatic trials to determine treatment effectiveness in real-world clinical practice, are indicated in this field.