["Journal of Child Psychology and Psychiatry, EarlyView. ", "\n\nBackground\nSocial motor synchrony is critical for successful social interaction. It remains unclear whether autistic children exhibit distinct differences in intentional versus spontaneous social motor synchrony, as well as what underlying interpersonal neural synchrony (INS) mechanisms drive these potential differences.\n\n\nMethod\nFifty‐four children (28 autistic) completed intentional (a delayed and synchronous imitation tasks in EX1) and spontaneous (a rhythmic hand‐clapping task in EX2) tasks with an adult. Brain signals were collected by a portable multichannel fNIRS device and classified by GaussianNB machine learning approach.\n\n\nResults\nCompared with non‐autistic children, autistic children showed: (1) significantly lower behavioral synchrony across both two experiments; (2) reduced activation in the right temporoparietal junction (r‐TPJ, CH18) during Ex1, with no significant group differences in activation observed across all 20 fNIRS channels during Ex2; (3) significantly lower INS values in task‐specific brain regions, that left inferior parietal lobule (l‐IPL, CH3) in the delayed imitation condition in EX1; left inferior frontal gyrus (l‐IFG, CH2), l‐IPL (CH9), and r‐TPJ (CH18) in the synchronous imitation condition in Ex1, and in the IPL (CH8, CH10‐14) and r‐TPJ (CH18) in Ex2. The GaussianNB model successfully discriminated between autistic and non‐autistic children using task‐related INS values, with classification accuracy varying by task condition, reaching 55.56% in the delayed imitation condition of EX1, 57.41% in the time‐lag analysis condition of EX1, 64.81% in the synchronous imitation condition of EX1, and 74.07% in Ex2. Notably, the SHAP toolkit identified key channels driving group distinction—and these channels fully overlapped with the statistically significant INS channels identified in the analyses.\n\n\nConclusions\nAutistic children exhibit differences in both intentional and spontaneous social motor synchrony, and these differences are linked to reduced INS in key social cognitive brain regions (IFG, IPL, TPJ). This research advances understanding of social functioning variations in autistic individuals and provides a foundational foundation for developing INS‐based diagnostic tools.\n\n"]