Purpose: To investigate the effects of Shared Decision-Making (SDM) using Routine Outcome Monitoring (ROM) primary on patients’ perception of Decisional Conflict (DC), which measures patients’ engagement in and satisfaction with clinical decisions, and secondary on working alliance and treatment outcomes. Method: Multi-centre two-arm matched-paired cluster randomised-controlled trial in Dutch specialist mental health care. SDM using ROM (SDMR) was compared with Decision-Making As Usual (DMAU). Outcomes were measured at baseline (T0) and 6 months (T1). Multilevel regression and intention-to-treat analyses were used. Post hoc analyses were performed on influence of subgroups and application of SDMR on DC. Results: Seven teams were randomised to each arm. T0 was completed by 186 patients (51% intervention; 49% control) and T1 by 158 patients (51% intervention, 49% control). DC, working alliance, and treatment outcomes reported by patients did not differ significantly between two arms. Post hoc analyses revealed that SDMR led to less DC among depressed patients (p = 0.047, d =− 0.69). If SDMR was applied well, patients reported less DC (SDM: p = 0.000, d = − 0.45; ROM: p = 0.021, d = − 0.32), which was associated with better treatment outcomes. Conclusion: Except for patients with mood disorders, we found no difference between the arms for patient-reported DC. This might be explained by the less than optimal uptake of this generic intervention, which did not support patients directly. Regarding the positive influence of a higher level of applying SDM and ROM on less DC and better treatment outcomes, the results are encouraging for further investments in patient-oriented development and implementation of SDMR.