["Developmental Science, Volume 29, Issue 5, September 2026. ", "\nABSTRACT\nEarly childhood is a sensitive period for brain development, during which sustained caregiving experiences may shape neural systems that support emotion regulation and social cognition. In a longitudinal study of N = 133 mother–child dyads, we examined whether children's attachment‐related behavioral patterns at age 3 were associated with functional connectivity during the viewing of attachment‐related video clips. We further tested whether mother–child behavioral synchrony observed at ages 1 and 3 predicted children's functional connectivity. Neural activity was measured using functional near‐infrared spectroscopy (fNIRS), targeting the prefrontal cortex (PFC), and temporoparietal junction (TPJ). Attachment‐related behaviors were assessed through a structured observational procedure and rated using the Brief Attachment Screening Questionnaire (BASQ), and mother–child synchrony was coded using the Dyadic Interaction Coding (DIC) system. Children in the BASQ‐derived insecure group exhibited greater intra‐hemispheric connectivity, whereas those in secure group showed stronger inter‐hemispheric connectivity. Mother–child synchrony at ages 1 and 3 did not independently predict later neural connectivity. Instead, mother–child synchrony at age 3 moderated the association between synchrony at age 1 and connectivity at age 3: synchrony at age 1 predicted reduced intra‐hemispheric MTG connectivity only when synchrony at age 3 was high. These findings suggest that sustained mother–child synchrony may contribute to the functional organization of neural systems supporting attachment‐related processing.\n"]