Background and Study Aim. The assessment instrument for gross motor skills in physical education is crucial in developing fundamental motor skills in elementary school students. This research aims to formulate an assessment tool for gross motor skills tailored to students aged 7-9, aligning with their specific characteristics. Material and Methods. This study adopts a research and development approach utilizing the 4D development procedure (Define, Design, Develop, Disseminate). It involves the validation process by five experts in motor skills and physical education, comprising faculty members from five distinct universities. The trial subjects encompass 310 elementary school students aged 7-9, while the instrument's effectiveness is evaluated by 35 individuals, including physical education teachers and students. Statistical analyses encompass content validity, criterion validity, test-retest reliability using Cronbach's alpha, and descriptive percentage analysis. Results. This research has produced a gross motor skills assessment instrument comprising 11 movement components: sit and stand up, rolling, one-leg stand, gallop, slide, jump, bouncing a ball, catching, throwing, kicking, and hitting. The content validity result is 1>0.763, criterion validity (rxy) > 0.304, Cronbach's alpha value for all items > 0.60, and the average perception assessment of instrument effectiveness > 93%, falling into the category of extremely high. Conclusions. The assessment instrument demonstrates high validity and reliability, effectively addressing the challenges associated with the need for assessments tailored to the evaluated subject's characteristics, objectives, and relevance. An easily comprehensible instrument, coupled with language accessibility, yields positive outcomes for physical education teachers in the assessment process.