Bronchial asthma is a common disease treated primarily with inhaled corticosteroids (ICS) and long-acting beta-2 agonists (LABA), yet optimal add-on therapy with long-acting muscarinic antagonists (LAMA) or leukotriene receptor antagonists (LTRA) remains unclear. This multicenter, randomized, open-label phase 2 screening trial compared the efficacy of adding tiotropium (LAMA) versus montelukast (LTRA) to ICS/LABA therapy in adult patients with poorly controlled asthma. Patients were randomly assigned to receive either tiotropium or montelukast for 12 weeks. Primary outcome was the change in forced expiratory volume in 1 second (FEV₁), secondary outcomes included changes in asthma control questionnaire (ACQ-5), asthma control test (ACT) scores, and peak expiratory flow (PEF). Ninety-four patients were analyzed. Over 12 weeks, the tiotropium group showed significant improvements in FEV₁ and PEF, while the montelukast group showed no improvement in respiratory function. Both groups, however, showed significant improvements in ACQ-5 and ACT. The primary endpoint, change in FEV₁ improved similarly in both groups (tiotropium 2.03 ± 0.74 to 2.10 ± 0.72 L vs montelukast 2.08 ± 0.69 to 2.13 ± 0.67 L; p = 0.736), while the tiotropium group exhibited a significantly greater improvement in PEF (tiotropium 5.41 ± 1.85 to 5.92 ± 1.93 L/s vs montelukast 5.58 ± 1.84 to 5.65 ± 1.82 L/s; p = 0.025). Subgroup analysis of patients with allergic rhinitis revealed the ACQ-5 score was significantly improved in the montelukast group (tiotropium 0.94 ± 0.64 to 0.68 ± 0.76 L/s vs montelukast 1.26 ± 1.15 to 0.53 ± 0.86 L/s; p = 0.002). Adding tiotropium to ICS/LABA therapy improves respiratory function, whereas adding montelukast enhances symptom control, particularly in patients with allergic rhinitis. These findings underscore the importance of personalized treatment approaches in asthma management by demonstrating that both tiotropium and montelukast confer distinct benefits according to patient characteristics, while also addressing a significant gap in the literature regarding optimal adjunctive therapies for adult asthma, given the scarcity of studies directly comparing these add-on strategies. Further research is needed to establish definitive guidelines for optimal add-on therapies in asthma.