People who suffer a first episode of psychosis experience higher levels of distress and suffering. Early intervention programs combine pharmacological and psychosocial strategies that include different components, such as cognitive-behavioural therapy, psychosocial interventions, medication adherence, family psychoeducation, counselling, etc. Among the complementary approaches, mindfulness-based interventions help participants to cultivate a radical acceptance of their psychotic experiences within a person-centered framework. They show promising results for people with longer duration of psychosis, but there is still no evidence for people who have recently experienced their first episode of psychosis. The present parallel-group, single-blind (evaluator), randomised (1:1 ratio), controlled (versus active comparator), superiority, clinical trial will compare the effectiveness of SocialMIND on social functioning as measured by the Personal and Social Performance (PSP) scale. The active comparator will be a psychoeducational multicomponent intervention (PMI) that incorporates elements of early intervention programs that are effective for people who have suffered a first episode of psychosis. Both SocialMIND and PMI encompass eight weekly sessions, four bi-weekly sessions, and five monthly sessions. Changes in primary and secondary outcomes will be measured after weekly (8th week), bi-weekly (16th week) and monthly sessions (56th week), and 3 months after completing the intervention (68th week). Secondary outcomes include symptoms of psychosis, anxiety and depression, as well as indicators of general functioning. Tertiary outcomes are measures of social cognition, neurocognition, mindfulness, and indicators of inflammation and oxidative stress. A final sample of 80 participants is proposed to detect clinically significant differences in social functioning. This is the first mindfulness-based social cognition training for people with psychosis. SocialMIND aims to generate changes in the real-life functioning of people who have experienced a first episode of psychosis, and to be at least as effective as a psychoeducational multicomponent program. Adherence to the interventions is a common problem among young people with psychosis, so several difficulties are anticipated, and some methodological issues are discussed. The trial was registered in ClinicalTrials.gov in October 2018 (NCT03309475).