Ptychostomum pseudotriquetrum, commonly known as marsh bryum,[2][3] is a species of moss belonging to the family Bryaceae.

[5][8] The stems branch sparingly by means of subfloral innovations,[8] which are elongate and evenly foliate.

[7] Some populations in the Northern Hemisphere[8] can reproduce asexually by means of brown gemmae in the leaf axils.

[9] The diplolepidous peristome is composed of two sets of teeth, an inner endostome and an outer exostome, which ring the mouth of the capsule.

[8][7] The operculum (the cap of the capsule) is 0.5–0.8 mm long and conic-apiculate, ending in a sharp point.

[5] The pale yellow or green spores measure 12–18 μm in diameter and are finely papillose.

[11] P. bimum has twice as many chromosomes as and probably evolved from P. pseudotriquetrum[12] As of June 2024, World Flora Online treats the two species as separate.

[7] P. pseudotriquetrum has a bipolar distribution and is additionally found in temperate regions in the Southern Hemisphere.

[14] It is found on all continents, including Antarctica,[8] but is largely absent from the tropics, subtropics, and islands in the central Pacific Ocean.