Fissidens dubius, commonly known as the rock-pocket moss, is a species of moss belonging to the family Fissidentaceae. First described by Palisot de Beauvois in 1805 from material collected in Philadelphia, it is a relatively large moss growing up to 5 cm (2 in) tall that forms loose to dense mats on calcareous substrates. The species is characterised by its serrated leaf margins, pale border of thick-walled cells, and typically dioicous reproduction. It has a broad global distribution across Europe, North and South America, Asia, North Africa, and New Zealand, occurring in various habitats from lowlands to alpine zones, particularly in shaded locations on basic soils and rocks. While morphologically similar to F. adianthoides, F. dubius is distinguished by its smaller leaf cells and irregularly thickened leaf tips. The species is classified as Least Concern due to its stable populations and absence of major threats.

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