Purpose The Simple Knee Value (SKV) is an outcome score in which patients are asked to grade their knee function as a percentage of that of a normal knee. The primary aim of this study was to validate the SKV by measuring its correlation with existing knee-related PROMs. Methods This was a prospective study conducted at a teaching hospital to assess the SKV's validity. The study enrolled 47 young patients (16-54 years old), 49 older patients (≥ 55 years) and 30 healthy controls. A paper questionnaire consisting of the Lysholm, IKDC, KOOS, WOMAC and SKV was given to subjects three times (enrolment, 1-month preoperative visit and 6 months postoperative visit). The criterion validity of the SKV was determined by correlating it to existing knee PROMs using the Spearman correlation coeicient (S). SKV test-retest reliability was assessed by the intraclass correlation coeicient (ICC) between two time points (initial consultation at enrolment and preoperative visit, relecting the same clinical condition). Responsiveness to change was determined by comparing the SKV scores before and after surgery (enrolment consultation and 6 months postoperative). Discriminative ability was determined by comparing the SKV distribution in patients and controls. Results There was a strong and signiicant correlation between the SKV and the gold standard Lysholm, IKDC, KOOS and WOMAC in the younger patients and the older patients (p < 0.0001). The reliability between the SKV at the initial consultation and before surgery was excellent (ICC 0.862, 95% CI 0.765; 0.921) in the younger patients, and moderate (ICC 0.506, 95% CI 0.265; 0.688) in the older patients. The SKV was responsive to change in both patient groups (p < 0.0001 for the SKV before versus 6 months after surgery). Like the other knee-speciic PROMs (p < 0.0001), the SKV was able to distinguish between patients and controls (p < 0.0001). Conclusions The SKV is valid as it is signiicantly correlated to existing knee PROMs. It is also reliable, responsive to change and discriminating. Its simplicity gives it many advantages and it can be used by physicians in their daily practice. Level of evidence Level II.