This study documents exceptionally well-preserved microbial biosignatures within Pleistocene reefal microbialites recovered from the Hawaiian Drowned Reefs during IODP Expedition 389, including calcified exopolymeric substances, microbial filaments, and putative cyanobacterial structures rarely preserved in marine carbonate environments. The remarkable quality of preservation provides direct evidence for the role of euphotic microbial mat communities in reef microbialite formation and offers a rare opportunity to reconstruct microbial community structure and environmental conditions through time. These findings highlight the value of reefal microbialites as archives of past microbial ecosystems and environmental change during Quaternary climate fluctuations.