This observational study aimed to examine the clinimetric properties of the Greek for Greece translation of the Western Ontario and McMaster Osteoarthritis Index (WOMAC^®). One hundred and twenty-three patients with knee osteoarthritis (mean age 69.5 years) participated in the study. An extensive reliability study was carried out to assess WOMAC’s internal consistency and repeatability (8-day interval). In addition, we examined the construct (convergent, nomological and known-groups) and criterion-related (concurrent and predictive) validity of the index against both self-report [SF-36 and combined visual analog/faces pain scale-revised (VAS/FPS-R)] and physical performance measures [timed up and go test (TUG)]. The internal consistency of the WOMAC subscales ranged from high (0.804) to excellent (0.956). Intra-class correlation coefficients for test–retest reliability were excellent, ranging from 0.91 to 0.95. Partial correlation analysis, adjusted for age and use of an assistive device, showed that WOMAC scores were significantly associated with all validation criteria, presenting fair to strong (−0.33 to −0.86) correlation coefficients. WOMAC-function was strongly associated with SF36-function (−0.86) and TUG (0.71), WOMAC-pain to VAS/FPS-R (0.71) and SF36-pain (−0.67). Of all WOMAC outcomes, stiffness subscale had the lowest, though still significant, correlations with all validation criteria. Multiple linear regression analyses indicated that WOMAC-function was a significant factor for TUG, WOMAC-pain for VAS/FPS-R and both for SF36-function and SF36-pain. The WOMAC LK3.1 Greek for Greece Index is a reliable and valid assessment tool for the evaluation of individuals with knee osteoarthritis, showing excellent reliability and significant validity properties.