The hippocampus (Hc) is composed of cytoarchitectonically distinct subfields: dentate gyrus (DG), cornu ammonis sectors 1-3 (CA1-3), and subiculum. Limited evidence suggests differential maturation rates across the Hc subfields. While longitudinal studies are essential in demonstrating differential development of Hc subfields, a prerequisite for interpreting meaningful longitudinal effects is establishing test–retest consistency of Hc subfield volumes measured in vivo over time. Here, we examined test–retest consistency of Hc subfield volumes measured from structural MR images in two independent developmental samples. Sample One (n = 28, ages 7–20 years, M = 12.64, SD = 3.35) and Sample Two (n = 28, ages 7–17 years, M = 11.72, SD = 2.88) underwent MRI twice with a 1-month and a 2-year delay, respectively. High-resolution PD-TSE-T2-weighted MR images (0.4 × 0.4 × 2 mm3) were collected and manually traced using a longitudinal manual demarcation protocol. In both samples, we found excellent consistency of Hc subfield volumes between the two visits, assessed by two-way mixed intraclass correlation (ICC (3) single measures ≥ 0.87), and no difference between children and adolescents. The results further indicated that discrepancies between repeated measures were not related to Hc subfield volumes, or visit number. In addition to high consistency, with the applied longitudinal protocol, we detected significant variability in Hc subfield volume changes over the 2-year delay, implying high sensitivity of the method in detecting individual differences. Establishing unbiased, high longitudinal consistency of Hc subfield volume measurements optimizes statistical power of a hypothesis test and reduces standard error of the estimate, together improving external validity of the measures in constructing theoretical models of memory development.