Sphagnum squarrosum, commonly known as the spiky bog-moss or spreading-leaved bog moss, is a peat moss species found in nutrient-rich, damp soils and wetlands across the Northern Hemisphere, with isolated populations in South America. Its spiky appearance, resulting from strongly spreading branch leaves, distinguishes it from other peat moss species. Playing an important role in wetland succession, the species is one of the first Sphagnum mosses to colonise developing wetlands. It shows considerable tolerance to mineral-rich conditions and actively modifies its habitat through cation exchange processes.

Unlike many other peat mosses that require highly acidic conditions, S. squarrosum thrives in areas with moderate calcium levels, particularly along stream banks, in fens, and in wet woodlands dominated by Salix (willows) and Betula (birches). It disperses efficiently, producing more spores per capsule and per patch than other Sphagnum species. It shows high genetic diversity across its range, having survived the Last Glacial Maximum in multiple European refugia. The species serves as the specific host for the fungal parasite Discinella schimperi, which infects about half of all documented populations.

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