Cough is a common pediatric complaint, resulting in frequent doctor's office or hospital visits. In Brazil, the leaves of chambá (Justicia pectoralis) have been used extensively in herbal preparations for the treatment of many respiratory problems such as cough, bronchitis, and asthma. Thus, this study aimed to assess the effectiveness of chambá syrup for the treatment of cough and respiratory symptoms in children. Between November 2017 and February 2018, patients aged 1 to 12 years with respiratory symptoms and cough, and onset of symptoms greater than 48 h were enrolled in this trial. Participants were randomized to receive either chambá syrup in three daily doses or placebo. Effectiveness was assessed through a questionnaire, to measure the intensity of cough and respiratory symptoms, before the intervention and 48 h after, using Likert scale responses. A total of 114 children, 56 in the chambá group and 58 in the placebo group, participated in the study. After 48 h, there was a significant reduction in frequency of cough, severity of cough, severity of nasal congestion, and in improvement in the ability to sleep for both the child and caregiver in the chambá group (p< .0001 for all variables); for participants in the placebo group, there were no significant differences after intervention, in comparison with baseline. In this study population, chambá syrup was effective in the treatment of cough and respiratory symptoms, providing an overall improvement of symptoms when compared to placebo, as well as improving the ability to sleep.