The Hippocampal Subfields Group (HSG) is an international organization seeking to develop a histologically-valid, reliable, and freely available segmentation protocol for high-resolution T2-weighted 3T MRI. Collaboration of multiple HSG working groups has produced a segmentation protocol that reached consensus by a modified Delphi procedure and passed reliability testing. The segmentation protocol is based on a novel histological reference data set labeled by multiple expert neuroanatomists, which ensured the validity of the protocol. All definitions were rated with high clarity and reached consensus agreement. The harmonized protocol includes labels for the internal boundaries between subiculum, Cornu Ammonis (CA) 1-3 fields, and dentate gyrus following a geometric heuristic that, when combined with the external boundaries that previously reached consensus, labels subfield volumes throughout the hippocampal body with high inter- and intra-rater reliability.