The Alberta Infant Motor Scale (AIMS) is a diagnostic tool for the assessment of the motor performance of infants from the time of birth, to the period of independent walking (0–18 months). This study aims to derive a Polish version of the AIMS through its cultural adaptation and validation. The study included 145 infants aged 0–18 months, who were divided into four further age groups: 0–3 months, 4–7 months, 8–11 months, and older than 12 months. The validation was based on an analysis of intrarater and interrater reliability values, as well as concurrent validity, using the gross motor scale of Peabody Developmental Motor Scales-2 (PDMS-2). The total Intraclass Correlation Coefficient (ICC) for intrarater reliability was 0.99 (ICC range in positions was 0.87–0.99, in subgroups was 0.91–0.99), while in particular positions, the ICC ranges were as follows: prone 0.97–0.99, supine 0.94–0.99, sitting 0.95–0.99, and standing: 0.63–0.99. The total ICC for interrater reliability was 0.99 (ICC range in positions was 0.98–0.99, in subgroups was 0.91–0.99), while in particular positions, the ICC ranges were as follows: prone 0.95–0.99, supine 0.93–0.96, sitting 0.93–0.98, standing 0.91–0.98. Only the standing position was analyzed for the subgroup of participants over 12 months old. The Spearman correlation between the Polish version of the AIMS and the gross motor scale of PDMS-2 was significant in the total population (r = 0.97,