Objective: To explore the contribution of a mindfulness-based intervention as an adjuvant to outpatient substance use disorder treatment. Outcomes included substance use behavior, depression and anxiety symptoms, and anger expression. Methods: This preliminary study for a pragmatic randomized controlled trial with stratified random allocation included three months of follow-up. In two outpatient clinics linked to public universities, a mindfulness-based intervention plus treatment as usual (experimental group n=22) was compared to treatment as usual (control group n=20). The study included data from self-report measurements and the patients’ records, which were evaluated according to intention-to-treat analysis through generalized estimating equations and generalized method of moments estimation. Results: The experimental group had lower symptoms of depression (b=-6.82; 95%CI -12.45 to -1.18) and anxiety (b=-0.25; 95%CI -0.42 to -0.09), and anger expression (b=-9.76; 95%CI -18.98 to -0.54) three months after the intervention. We detected no effect on substance use behavior. Conclusion: The mindfulness-based intervention yielded promising results as an adjuvant to outpatient substance use disorder treatment, since it reduced levels of highly prevalent symptoms in this population. However, further studies with longer follow-up periods and larger samples are required.