To examine reliability and validity of the new Social Motor Function Classification System for Children with Autism Spectrum Disorders (SMFCS-ASD). The SMFCS-ASD reliability was examined on 25 children (62.4 months SD 7.8) with ASD among six physical therapists. The validity study involved 1001 children (57.0 months, SD 9.9) with ASD using the gross motor scale (GMS) of the Peabody Developmental Motor Scales (PDMS-2). The indices of agreement and reliability across six examiners were moderate to substantial (Cohen’s κ ≤ 0.65 and ICC > 0.90, all p < 0.001). The SMFCS-ASD was significantly correlated with the GMS of PDMS-2 (all rho from 0.61 to 0.76, p < 0.001). The SMFCS-ASD was reliable and significantly correlated with the GMS of the PDMS-2.