Wide local excision of the primary tumour is the mainstay of treatment for melanoma patients. The aims of this study were to assess the patient- and observer-reported long-term scar quality after surgery using the patient and observer scar assessment scale (POSAS) in melanoma patients, to assess the reliability and validity of POSAS, and to identify factors influencing the scar assessment. This cross-sectional clinical study included 320 melanoma patients with primary tumours on the trunk and limbs. Data regarding patients, treatment, scar characteristics and functional outcomes was analysed. Internal consistency, inter-rater reliability, and convergent validity were examined. Factors influencing the patient- and observer-reported scar quality were tested using regression analyses. Results of the POSAS showed an overall good scar quality. The internal consistency of POSAS was good, and the convergent validity was strong. The inter-rater reliability was only moderate. The patients were influenced by the POSAS sub-items: colour, irregularity, thickness and pain. The observer was influenced by the POSAS sub-items: vascularity, surface area, thickness, relief and pliability. Both patient- and observer-reported scar qualities were influenced by age, location, type of superficial suture, keloids and widened scars. Moreover, the patients were influenced by the scar tightness while the observer was influenced by postoperative complications, hypertrophic scars, suture marks and dog ears. In conclusion POSAS is a reliable and valid scar assessment tool. The factors influencing patient- and observer-reported scar quality differed, and better understanding of this may improve treatment and hence patient-reported scar quality.