Cough is a frequent symptom associated to upper respiratory tract infections (URTIs) and, although being self-limiting, it might deeply affect the quality of life. Homeopathic products are often employed by patients to treat cough, but the evidence on their efficacy is scarce. Thus, we tested the efficacy of a homeopathic syrup in treating cough arising from URTIs with a randomized, double blind, placebo controlled clinical trial. Patients were treated with either the homeopathic syrup or a placebo for a week, and recorded cough severity in a diary by means of a verbal category-descriptive score for two weeks. Sputum viscosity was assessed with a viscosimeter before and after 4 days of treatment; patients were also asked to provide a subjective evaluation of viscosity. Eighty patients were randomized to receive placebo (n = 40) or the homeopathic syrup (n = 40). All patients completed the study. In each group cough scores decreased over time, however, after 4 and 7 days of treatment, cough severity was significantly lower in the homeopathic group than in the placebo one (p < 0.001 and p = 0.023, respectively). Sputum was collected from 53 patients: in both groups its viscosity significantly decreased after 4 days of treatment (p < 0.001); however, viscosity was significantly lower in the homeopathic group (p = 0.018). Instead, the subjective evaluation did not significantly differ between the two groups (p = 0.059). No adverse events related to any treatment were reported. We concluded that the homeopathic syrup employed in the study was able to effectively reduce cough severity and sputum viscosity, thereby representing a valid remedy for the management of acute cough induced by URTIs.