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Marine radiaxial fibrous calcites are common pore-filling carbonate fabrics in Palaeozoic and Mesozoic carbonates, yet recent analogues are scarce. Although ancient marine radiaxial fibrous calcites are often considered a cement phase and used for palaeoenvironmental interpretations, their origin (primary precipitation versus diagenetic alteration of a pre-existing fabric) and their specific formation processes remain ambiguous. This may be related to insufficient information from a few recent analogues and limitations of traditional methods in investigating ancient radiaxial fibrous calcites. This study documents a Holocene occurrence of marine pore-filling radiaxial fibrous Mg calcites and focuses on their origin, formation processes and implications for ancient analogues. The radiaxial fibrous Mg calcites examined in this study share morphological and mineralogical similarities with numerous ancient marine radiaxial fibrous calcites with a Mg calcite precursor. Based on petrological and geochemical data, the radiaxial fibrous Mg calcites are interpreted as secondary porewater products formed during early burial alteration of micritic Mg calcite precursor cement. These findings provide implications for, at least in part, ancient marine radiaxial fibrous calcites in the context of the formation process and mechanism and are relevant for palaeoceanographic and palaeoenvironmental interpretations based on the presence of this carbonate phase.