Objective: This study aims to evaluate the effectiveness of medication therapy combined with transcranial Direct Current Stimulation (tDCS) in improving problem-solving and emotion regulation abilities of patients with bipolar disorder (BD) type I. Methods: This is a randomized clinical trial conducted on 30 patients with BD I, randomly assigned into two groups of Medication (n = 15, receiving mood stabilizers including 2−5 tablets of lithium 300 mg, sodium valproate 200 mg, and carbamazepine 200 mg) and Medication + tDCS (n = 15, receiving mood stabilizers plus tDCS with 2 mA intensity over the right dorsolateral prefrontal cortex for 10 days, two sessions per day each for 20 minutes). The Tower of London (TOL) test and Emotion Regulation Questionnaire (ERQ) were used for assessments before, immediately, and 3 months after interventions. Results: There was a significant difference between groups in total ERQ (p = 0.001) and its cognitive reappraisal domain (p = 0.000) which were increased, but the difference was not significant in its expressive suppression domain (p ＞ 0.05). After 3 months, their level decreased. In examining problem-solving variable, the combined therapy could significantly reduce only the total number of errors under TOL test (p = 0.00), but it remained unchanged after 3 months. Conclusion Medication therapy plus tDCS is effective in improving problem-solving and emotional regulation (cognitive reappraisal) skills of patients with BD I.