Fundamental motor skills (FMS) are essential for enjoyable, confident and skillful participation in physical activity across the lifespan. Due to the alarming low level of FMS proficiency in children and adolescents worldwide, the development of motor competency is an urgent issue for physical education. The promotion and implementation of a systematic process of teaching and learning FMS should be a physical education priority. Accordingly, effective assessment tools for evaluating FMS should be adopted or developed. Because FMS assessment for both children and adolescents need further effective solutions, the primary aim of this study was to develop the new age-related test of FMS (Fundamental Motor Skills in Sport test, in Polish: Test Fundamentalnych Umiejętności Ruchowych w Sporcie, FUS). The secondary aim of this study was to establish validity and inter-rater, intra-rater, test-retest reliabilities and internal consistency of the FUS test. The FUS test involves six sport skill-based tasks: hurdling, jumping rope, forward roll, ball bouncing, throwing and catching a ball, and kicking and stopping a ball. Two hundred sixty-four Polish students in grades 1–3 (7–9 yrs; n = 81), 4–6 (10–12 yrs; n = 89) and 7–8 (13–14 yrs; n = 94), including 139 girls and 125 boys completed the FUS test. The content validity index for all items was notably high. Both inter-rater and intra-rater reliability showed substantial to almost perfect agreement, with observed agreements for FUS skills between 78.5 and 93.1%. Ball bouncing had a moderate correlation with the forward roll and throwing and catching, while other correlations were low or insignificant. ICC values, ranging from 0.95 to 0.97, confirmed excellent test-retest reliability. The results of our study provide evidence that the FUS test is valid, reliable, and feasible to administer in school settings. Therefore, this tool test has the potential to support deliberate practice and improve motor competence by providing a standardized and structured approach to measuring FMS among school-aged children and adolescents.