Many patients with schizophrenia have negative symptoms, but their evaluation is a challenge. Thus, standardized assessments are needed to facilitate identification of these symptoms. Many tools have been developed, but most are based on observer ratings. Self-evaluation can provide an additional outcome measure and allow patients to be more engaged in their treatment. The aim of this study was to present a novel tool, Self-evaluation of Negative Symptoms (SNS), and demonstrate its validity. Forty-nine patients with schizophrenia and schizoaffective disorders according to DSM-5 were evaluated. Cronbach’s coefficient (α = 0.867) showed good internal consistency. Factor analysis extracted 2 factors (apathy and emotional) that accounted for 75.2% of the variance. The SNS significantly correlated with the Scale of Assessment of Negative Symptoms (r = 0.628) and the Clinician Global Impression on the severity of negative symptoms (r = 0.599), supporting good convergent validity. SNS scores did not correlate with level of insight (r = 0.008), Parkinsonism (r = 0.175) or Brief Psychiatric Rating Scale positive subscores (r = 0.253), which indicates good discriminant validity. The intrasubject reliability of the SNS revealed excellent intraclass correlation coefficients (ICC = 0.942). Taken together, the results show that the SNS has good psychometric properties and satisfactory acceptance by patients. The study also demonstrates the ability of patients with schizophrenia to accurately report their own experiences. Self-assessments of negative symptoms should be more widely employed in clinical practice because they may allow patients with schizophrenia to develop appropriate coping strategies.