BACKGROUND: Chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) is common, yet no curative treatment identified. Cinnamon is a herbal substance, which has many applications in medicine. AIM: The aim of the study was to study the effect of cinnamon on patients with chronic pelvic pain syndrome. METHODS: Sixty patients with documented CP/CPPS randomized into two groups during 2018 and 2019 in Baghdad. The first group received 60 capsules each contained 1 g of cinnamon. The other group received 60 capsules each contained 1 g of sugar powder (placebo). All the patients instructed to take one capsule twice daily for 1 month. National Institutes of Health-Chronic Prostatitis Symptom Index (NIH-CPSI) was reported for both groups at baseline and after 1 month of treatment. The primary outcome was a patient perceivable improvement defined as a reduction of the NIH-CPSI by 6 or more points after 1 month, whereas improvement of sub-scores of NIH-CPSI (pain, urinary symptoms, and quality of life) considered as a secondary outcome, and adverse reactions reported. RESULTS: Thirteen patients (43.3%) of the cinnamon group have 6 or more points of reduction in the total NIH-CPSI compared to four patients (13.3%) of the control groups (p = 0.01). The improvement in total NIH-CPSI score was mainly due to improvement in pain sub-score, whereas in urinary symptoms, there was marginal change with no significant change in the quality of life score. The only reported side effect was gastric upset in one patient. CONCLUSION: The study concluded that cinnamon improves NIH-CPSI in patients with CP/CPPS. REGISTRATION: The study was registered on ClinicalTrials.gov with the ID: NCT03946163.