To investigate the relationship between the posterior cingulate gyrus and neurobehavioral disorders (ND) in neonates with hypoxic-ischemic encephalopathy (HIE) using resting-state functional magnetic resonance imaging (rs-fMRI) combined with ultrasound. Methods: A total of 120 neonates with HIE were enrolled and divided into a non-ND group (n=72) and an ND group (n=48) based on the results of the Neonatal Behavioral Neurological Assessment (NBNA) scores. The ultrasound gray-scale values of the posterior cingulate gyrus and the rs-fMRI findings were compared between the two groups. Logistic regression analysis was conducted to identify independent influencing factors for ND in neonates with HIE. Multiple linear regression analysis was performed to explore the correlation between the amplitude of low-frequency fluctuations (ALFF) values of the posterior cingulate gyrus and global network property parameters of the brain. The dose-response relationship between each indicator and ND in neonates with HIE was analyzed using the restricted cubic spline method. Receiver operating characteristic (ROC) curve analysis was utilized to assess the relationship between the ultrasound gray-scale values of the posterior cingulate gyrus, the severity of HIE, and ND. Results: Compared with the non-ND group, the ND group exhibited a lower 5-minute Apgar score at birth (P<0.05) and a significantly higher degree of HIE (P<0.05). The ALFF values of the posterior cingulate gyrus, globus pallidus, and parahippocampal cortex were decreased (P<0.05). Network strength (Sp), local efficiency (Eloc), and normalized shortest path length ( λ) were reduced (P<0.05). The ultrasound gray-scale values of the posterior cingulate gyrus were significantly increased (P<0.05). Logistic regression analysis revealed that severe HIE, as well as lower levels of ALFF values, Sp, Eloc, and λ in the posterior cingulate gyrus, were independent influencing factors for ND in neonates with HIE (P<0.05). Multiple linear regression analysis indicated a positive correlation between the ALFF values of the posterior cingulate gyrus and the global network property parameters Sp, Eloc, and λ (β >0, P<0.05). The restricted cubic spline analysis demonstrated a significant negative correlation between the ALFF values, Sp, Eloc, and λ of the posterior cingulate gyrus and the occurrence of ND in neonates with varying degrees of HIE. ROC curve analysis showed that the ultrasound gray-scale values of the posterior cingulate gyrus had high diagnostic value for assessing the severity of HIE (AUC=0.871, P<0.05) but no significant relationship with the occurrence of ND (AUC=0.634, P>0.05). Conclusion: Neonates with HIE exhibit decreased ALFF values and abnormal functional connectivity in the posterior cingulate gyrus, which are associated with an increased risk of ND.