The six-item Positive and Negative Syndrome Scale (PANSS-6) is a measure of the severity of core symptoms of schizophrenia, which can be administered via the brief Simplified Negative and Positive Symptoms Interview (SNAPSI). A recent study has confirmed the validity of PANSS-6 ratings as derived by SNAPSI (PANSS-6SNAPSI) among inpatients with schizophrenia. We aimed to test the validity of PANSS-6SNAPSI among outpatients with schizophrenia using PANSS-6 ratings extracted from the 30-item PANSS-30 as derived by the Structured Clinical Interview for the Positive and Negative Syndrome Scale (PANSS-6SCI-PANSS) as a gold standard reference. PANSS-6SNAPSI and PANSS-6SCI-PANSS ratings were obtained at two time points by independent raters with established inter-rater reliability. Agreement between PANSS-6SNAPSI and PANSS-6SCI-PANSS ratings was estimated via intra-class coefficients (ICCs) and responsiveness over time was quantified using Spearman's rank correlation coefficients. Post hoc "leave-one-out" analyses were carried out, in which each rater in turn was excluded from the ICC calculations. Seventy-three outpatients with schizophrenia participated in the study (mean age: 38.3 years; 56% males). The ICC for PANSS-6SNAPSI versus PANSS-6SCI-PANSS was 0.67 [95%CI = 0.56-0.76] and the Spearman's rank correlation coefficient for responsiveness was 0.40 (p = 0.004). When data from a specific outlying rater were excluded, the ICC for PANSS-6SNAPSI versus PANSS-6SCI-PANSS was 0.75 [95% CI = 0.63-0.83] and the Spearman's rank correlation coefficient for responsiveness was 0.55 (p = 0.018). We found PANSS-6SNAPSI ratings to have acceptable clinical validity, suggesting that PANSS-6SNAPSI can be used for both inpatients and outpatients with schizophrenia.