Although serotonin reuptake inhibitors are recommended as first-line agents for major depressive disorder, delayed onset of action is problematic, and faster effective treatment is needed. Sulpiride, a dopamine-mediated agent, has been reported to show faster antidepressant efficacy, and we examined the efficacy of adjunctive sulpiride in combination with paroxetine (PAX), compared with PAX alone, to clarify whether the combined treatment exerts faster effect. Forty-one major depressive disorder patients were enrolled in this 12-week open-label trial and were randomly assigned to a PAX (10-40 mg/d) or a PAX (10-40 mg/d) plus sulpiride (100 mg/d) group. Assessments included the Montgomery-Asberg Depression Rating Scale, the 17-item Hamilton Rating Scale for Depression, and the Zung Self-rating Depression Scale on an intent-to-treat basis, and safety was also monitored. Thirty-three patients completed the study. Both PAX + sulpiride and PAX treatments showed a mean reduction in the total Montgomery-Asberg Depression Rating Scale score of 34.4 to 5.6 and 32.2 to 10.4, respectively (P < 0.001). The combined treatment group had a significantly superior outcome in terms of the change in the total Montgomery-Asberg Depression Rating Scale, Hamilton Rating Scale for Depression, and Zung Self-rating Depression Scale scores between week 1 and the study end point (P < 0.05). Median times to response among responders alone for the combined treatment and monotherapy were 2 and 6 weeks, respectively. Both treatments were well tolerated, with no clinically significant differences in safety measures except for an elevation of prolactin in the combined treatment group. The combination treatment may be a safe and effective strategy for accelerating antidepressant response.