Background Skin tears are acute wounds that are frequently misdiagnosed and under-reported. A standardized and globally adopted skin tear classification system with supporting evidence for diagnostic validity and reliability is required to allow assessment and reporting in a consistent way. Objectives To measure the validity and reliability of the International Skin Tear Advisory Panel (ISTAP) Classification System internationally. Methods A multicountry study was set up to validate the content of the ISTAP Classification System through expert consultation in a two-round Delphi procedure involving 17 experts from 11 countries. An online survey including 24 skin tear photographs was conducted in a convenience sample of 1601 healthcare professionals from 44 countries to measure diagnostic accuracy, agreement, inter-rater reliability and intrarater reliability of the instrument. Results A definition for the concept of a 'skin flap' in the area of skin tears was developed and added to the initial ISTAP Classification System consisting of three skin tear types. The overall agreement with the reference standard was 0.79 [95% confidence interval (CI) 0.79-0-80] and sensitivity ranged from 0.74 (95% CI 0.73-0-75) to 0.88 (95% CI 0.87-0.88). The inter-rater reliability was 0.57 (95% CI 0.57-0.57). The Cohen's Kappa measuring intrarater reliability was 0.74 (95% CI 0.73-0.75). Conclusions The ISTAP Classification System is supported by evidence for validity and reliability. The ISTAP Classification System should be used for systematic assessment and reporting of skin tears in clinical practice and research globally.