Purpose To develop and validate a novel score to more objectively assess the performance of diagnostic knee arthroscopy using a simulator. Methods A Diagnostic Arthroscopy Skill Score (DASS) was developed by ten AGA (AGA-Society for Arthroscopy and Joint-Surgery) instructors for the assessment of arthroscopic skills. DASS consists of two parts: the evaluation of standardized diagnostic knee arthroscopy (DASS part1 ) and the evaluation of manual dexterity, including ambidexterity and triangulation, using objective measurement parameters (DASS part2 ). Content validity was determined by the Delphi method. One hundred and eleven videos of diagnostic knee arthroscopies were recorded during simulator training courses and evaluated by six specially trained instructors using DASS. Construct validity, measurement error calculated by the minimum detectable change (MDC), internal consistency using Cronbach's alpha and interrater and intrarater reliability were assessed. The Bland-Altman method was used to calculate the intrarater agreement. Results Six skill domains were identiied and evaluated for each knee compartment. DASS, DASS part1, and DASS part2 showed construct validity, with experts achieving signiicantly higher scores than competents and novices. MDC was 4.5 ± 1.7 points for DASS part1 . There was high internal consistency for all domains in each compartment from 0.78 to 0.86. The interrater reliability showed high agreement between the six raters (ICC = 0.94). The evaluation of intrarater reliability demonstrated good and excellent agreement for ive raters (ICC > 0.80) and moderate agreement for one rater (ICC = 0.68). The Bland-Altman comparison showed no diference between the irst and second evaluations in ive out of six raters. Precision, estimated by the regression analysis and comparison with the method of Bland and Altman, was excellent for four raters and moderate for two raters. Conclusions The results of this study indicate good validity and reliability of DASS for the assessment of the surgical performance of diagnostic knee arthroscopy during simulator training. Standardized training is recommended before arthroscopy surgery is considered in patients. Level of evidence II.