Background Mounting evidence suggests that perivascular spaces (PVSs) visible at MRI reflect the function of the glymphatic system. Understanding PVS burden in neonates may guide research on early glymphatic-related pathologic abnormalities. Purpose To perform a visual and volumetric evaluation of PVSs that are visible at MRI in neonates and to evaluate potential associations with maturation, sex, and preterm birth. Materials and Methods In this retrospective study, T2-weighted brain MRI scans in neonates from the Developing Human Connectome Project were used for visual grading (grades 0-4) of PVSs in the basal ganglia (BG) and white matter (WM) and for volumetric analysis of BG PVSs. The BG PVS fraction was obtained by dividing the BG PVS volume by the deep gray matter volume. The association between postmenstrual age at MRI and BG PVS burden was evaluated using linear regression. PVS burden was compared according to sex and preterm birth using the Mann-Whitney test. Results A total of 244 neonates were evaluated (median gestational age at birth, 39 weeks; IQR, 6 weeks; 145 male neonates; 59%), including 88 preterm neonates (median gestational age at birth, 33 weeks; IQR, 6 weeks; 53 male neonates; 60%) and 156 term neonates (median gestational age at birth, 40 weeks; IQR, 2 weeks; 92 male neonates; 59%). For BG PVSs, all neonates showed either grade 0 (90 of 244; 37%) or grade 1 (154 of 244; 63%), and for WM PVSs, most neonates showed grade 0 (227 of 244; 93%). The BG PVS fraction demonstrated a negative relationship with postmenstrual age at MRI (r = -0.008; P < .001). No evidence of differences was found between the sexes for BG PVS volume (P = .07) or BG PVS fraction (P = .28). The BG PVS volume was smaller in preterm neonates than in term neonates (median, 45.3 mm3 [IQR, 15.2 mm3] vs 49.9 mm3 [IQR, 21.3 mm3], respectively; P = .04). Conclusion The fraction of perivascular spaces (PVSs) in the basal ganglia (BG) was lower with higher postmenstrual age at MRI. Preterm birth affected the volume of PVSs in the BG, but sex did not. © RSNA, 2023 Supplemental material is available for this article. See also the editorial by Huisman in this issue.