Objective To compare resting-state brain functional connectivity among neonates of different gestational ages using functional near-infrared spectroscopy (fNIRS). Methods Forty hospitalized neonates were enrolled and categorized by gestational age into very preterm (n=10), moderate-to-late preterm (n=10), and full-term (n=20) groups. At a corrected gestational age of (40 ± 4) weeks, resting-state whole-brain oxygenation signals were acquired with fNIRS. The frontal, parietal, and temporal lobes were defined as regions of interest (ROI). Functional connectivity strength was calculated, and between-group comparisons were performed at both the whole-brain and ROI levels. Results At the whole-brain level, the mean functional connectivity strength in preterm infants was significantly lower than that in full-term infants (0.55 ± 0.20 vs 0.68 ± 0.16, P=0.030). Subgroup analysis showed the lowest connectivity strength in the very preterm group (0.46 ± 0.19), significantly lower than in the moderate-to-late preterm group (0.64 ± 0.19, P=0.030) and the full-term group (0.68 ± 0.16, P<0.01). At the ROI level, the very preterm group exhibited significantly reduced connectivity relative to the full-term group across multiple key connections, including left frontal-right frontal, left temporal-right frontal, bilateral frontal-bilateral parietal (both ipsilateral and contralateral), and left parietal-right parietal connections (all P<0.05). Conclusions Preterm birth exerts a gestational age-dependent gradient effect on resting-state brain functional connectivity, with smaller gestational age associated with more pronounced reductions. Very preterm neonates show widespread decreases in connectivity at the whole-brain level and between key cortical regions.