Introduction An important proportion of asthma patients remain uncontrolled despite using inhaled corticosteroids and long-acting beta-agonists. Some add-on therapies, such as tiotropium bromide have been recommended for this subgroup of patients. The purpose of this study was to assess the cost-effectiveness of tiotropium as add-on therapies to ICS + LABA for children and adolescents with uncontrolled allergic asthma. Methods A probabilistic Markov model was created to estimate the cost and quality-adjusted life-years (QALYs) of patients with severe asthma in Colombia. Total costs and QALYS of two interventions including standard therapy (ICS + LABA), add-on therapy with tiotropium, were calculated over a time horizon from 6 to 18 years. Probability sensitivity analyses were conducted. Cost-effectiveness was evaluated at a willingness-to-pay value of $19,000. Results The model suggests a potential gain of 0.51 QALYs per patient per year on tiotropium, with a difference of US$265 per patient year with respect to standard therapy. The incremental cost-effectiveness ratio estimated was U$ 1928 in the deterministic model and US$2,017 in the probabilistic model after Monte-Carlo simulation. Our base‐case results were robust to variations in all assumptions and parameters. Conclusion Add-on therapy with tiotropium was cost-effective when added to usual care in children and adolescents with severe asthma who remained uncontrolled despite treatment with medium or high-dose ICS/LABA. Our study provides evidence that should be used by decision-makers to improve clinical practice guidelines and should be replicated to validate their results in other middle-income countries.