Personality disorder (PD) is increasingly categorized according to its severity, but there is no simple way to screen for severity according to ICD-11 criteria. We set out to develop the Standardized Assessment of Severity of Personality Disorder (SASPD). A total of 110 patients completed the SASPD together with a clinical assessment of the severity of personality disorder. We examined the predictive ability of the SASPD using the area under the ROC curve (AUC). Two to four weeks later, 43 patients repeated the SASPD to examine reliability. The SASPD had good predictive ability for determining mild (AUC = 0.86) and moderate (AUC = 0.84) PD at cut points of 8 and 10, respectively. Test-retest reliability of the SASPD was high (intraclass correlation coefficient = 0.93, 95% CI [0.88, 0.96]). The SASPD thus provides a simple, brief, and reliable indicator of the presence of mild or moderate PD according to ICD-11 criteria.