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The Rett Syndrome Gross Motor Scale (RSGMS) is an observational measurement, assessing gross motor skills in individuals with Rett syndrome. A Dutch version is lacking. The current study aims to translate and cross-culturally adapt the original RSMGS to Dutch and assess its inter-rater and intra-rater reliability. Translation and cross-cultural adaptation were performed in concordance with internationally accepted guidelines. A pretest was performed, and expert validation was assured. Video data of three girls with Rett syndrome was independently assessed by 27 physiotherapists via an online webinar to measure inter-rater reliability. Additionally, videos of 17 individuals with Rett syndrome were scored twice by two raters to assess intra-rater reliability. The reliability of the total score, the three subscale scores and the new items were analyzed using Intraclass Correlation Coefficients (ICC). Good comprehensibility and expert validation of the RSMGS-NL was achieved, and four items were added to the original scale. Inter-rater reliability for the total score was excellent (ICC 0.97, 95% CI 0.89–0.99), and good to perfect inter-rater reliability for the three subscales; Sitting, Standing & walking and Challenge was found (ICC values 1.0 (95% CI 0), 0.98 (95% CI 0.91–0.99) and 0.82 (95% CI 0.93–0.99) respectively). The intra-rater reliability was excellent for the total test score (ICC 0.98, 95% CI 0.97–0.99) and good to excellent for the subscale scores (ICC values 0.87 (95% CI 0.75–0.94), 0.99 (95% CI 0.98–1.0) and 0.97 (95% CI 0.95–0.99) respectively). The four new items (Standing to sitting, walking down a slope, ascending the stairs, descending the stairs) showed good to excellent intra-rater reliability. The RSGMS-NL is a reliable measure of gross motor skills in Dutch individuals with Rett syndrome. [ABSTRACT FROM AUTHOR]