Background: Currently available patient-reported outcome measures (PROM) have limited ability to assess unique issues related to lateral canthal lines (LCL). This study aimed to develop a PROM to assess the severity and psychosocial impact of LCL in afflicted patients. Methods: We conducted a cross-sectional survey at a tertiary hospital and two local clinics in Korea. Exploratory factor analysis was conducted to identify the underlying factor structure of the Facial Line Distress Scale for LCL (FINE-LCL), and the internal consistency and test-retest reliability were also examined. Results: We developed a questionnaire comprising 20 items in four domains. Coefficient alphas ranged from 0.94 to 0.97 for subdomains and 0.97 for the total questionnaire. The test-retest intraclass correlation coefficient ranged from 0.77 to 0.90. The FINE-LCL was moderately correlated with the appearance appraisal score and body image. Analysis of the accuracy of the FINE-LCL score in predicting severe LCL was characterized by an area under the curve of 0.79. Conclusions: FINE-LCL is a reliable, valid, and comprehensive PROM for the assessment of the severity of LCL and their associated distress.