Sphagnum platyphyllum, the flat-leaved bogmoss, is a species of moss in the family Sphagnaceae. It is a circumpolar peat moss found in arctic and subarctic regions of Europe and Asia, with populations also occurring in North America. The species typically grows in loose mats in wet, moderately nutrient-rich environments, particularly in areas dominated by sedges and locations that experience seasonal flooding. First described as a variety in 1875 and elevated to species status in 1884, it belongs to the subgenus Subsecunda within the "Pacific Rim clade". While most populations are haploid, the species shows evidence of a complex evolutionary history, including rare allodiploid populations. Although widely distributed and classified as a least-concern species globally, it faces varying levels of conservation concern across its range, being critically endangered in several European countries due to habitat degradation, peat extraction, and climate change. The species is distinctive among peat mosses for its poorly developed capitula with protruding stem buds, and its dioecious reproduction, with male plants and spore-producing structures being rare in many regions.

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