This randomized, multicenter, double-blind study was designed to compare specifically the onset of antidepressant action of mirtazapine orally disintegrating tablets (ODT) with venlafaxine extended-release (XR) formulation in outpatients with major depression. Both treatments were administered in a rapidly escalating dosing regimen. Target doses (mirtazapine ODT, 45 mg OD; venlafaxine XR, 225 mg OD) were reached by day 6 of treatment. On the primary efficacy parameter [the average of the change in HAM-D (17-item) total score on days 5, 8, 11, and 15], mirtazapine ODT was significantly superior to venlafaxine XR (P = 0.008). In addition, calculating the HAM-D score without the sleep items resulted in significant reductions in favor of mirtazapine ODT on days 8 (P = 0.006) and 11 (P = 0.037). The proportion of responders (HAM-D decrease of ≥50% from baseline) was higher in the mirtazapine ODT group on all assessment days, being significant on days 8 (P = 0.002), 11 (P = 0.004), and 22 (P = 0.027). More patients in the mirtazapine ODT group achieved remission (HAM-D total score of ≤7) up to day 29, and the difference was statistically significant on day 15 (P = 0.016). Significant differences in favor of mirtazapine ODT were evident in the CGI of change on days 8 (P = 0.019), 11 (P = 0.004), and 15 (P = 0.031), and the CGI of severity on days 8 (P = 0.014) and 11 (P = 0.033). Both treatments were well tolerated. These results indicate that mirtazapine ODT has a faster onset of antidepressant efficacy than venlafaxine XR in patients with major depressive disorder, and that this effect is independent of its sleep-improving properties.