Treatment-resistant severe mental illness (SMI) represents a major clinical challenge. The ketogenic diet (KD), widely recognized for its efficacy in pharmacoresistant epilepsy, has recently emerged as a potential metabolic intervention in psychiatric care. Preclinical evidence and case reports suggest that KD may improve psychotic and affective symptoms through neurobiological and metabolic mechanisms. However, studies conducted in outpatient settings and within Spanish-speaking populations remain scarce. We implemented a 9-week KD protocol in outpatients with severe refractory symptoms (N = 6). The dietary plan included weekly monitoring of ketosis. A repeated-measures pre–post design was employed. Psychotic and affective symptoms were assessed using the Symptom Checklist-90-R (SCL-90-R), a self-report measure of psychopathological symptoms. Post-intervention analyses revealed statistically significant reductions in Global Severity Index (W = 21.0, p = .031), Positive Symptom Total (W = 21.0, p = .031), Positive Symptom Distress Index (W = 21.0, p = .031), and Hostility (W = 21.0, p = .036). No serious adverse events were reported, and adherence was satisfactory. These preliminary findings suggest that implementing KD in refractory psychosis is feasible, safe, and potentially beneficial in outpatient populations. This study provides initial evidence within a Spanish-speaking context and supports the need for controlled trials with larger samples.