Demersatheca is a genus of extinct vascular plants of the Early Devonian (Pragian, around 410 million years ago).

The plant had smooth leafless stems at least 1 mm in diameter, but only regions which bore spore-forming organs or sporangia are well-known.

A particular feature of Demersatheca which distinguishes it from other zosterophylls is that the stalk-less sporangia were sunken into the stem of the spike, so that the outer valve was flush with the surface.

[1] D. contigua was initially called Zosterophyllum contiguum by Li and Cai, based on parts of two spikes (strobili) of sporangia.

When further specimens were found, a review by Li and Edwards agreed that the plant was a zosterophyll, but decided that the sunken sporangia and the absence of sporangial stalks were sufficient to require a new genus, Demersatheca.