Background: Cannabis use has a negative impact on psychosis. Studies are needed to explore the efficacy of psychological interventions to reduce cannabis use in psychosis. Our aim is to study the efficacy of a specific motivational intervention on young cannabis users suffering from psychosis. Methods: Participants (aged less than 35 years) were randomly assigned to treatment as usual (TAU) alone, or treatment as usual plus motivational intervention (MI + TAU). TAU was comprehensive and included case management, early intervention and mobile team when needed. Assessments were completed at baseline and at 3, 6 and12 months follow-up. Results: Sixty-two participants (32 TAU and 30 MI + TAU) were included in the study. Cannabis use decreased in both groups at follow-up. Participants who received MI in addition to TAU displayed both a greater reduction in number of joints smoked per week and greater confidence to change cannabis use at 3 and 6 months follow-up, but differences between groups were nonsignificant at 12 months. Conclusions: MI is well accepted by patients suffering from psychosis and has a short-term impact on cannabis use when added to standard care. However, the differential effect was not maintained at 1-year follow-up. MI appears to be a useful active component to reduce cannabis use which should be integrated in routine clinical practice.