Trithuria occidentalis is a species of plant in the family Hydatellaceae endemic to Western Australia.

It exhibits sexual dimorphism in respect to the number of bracts present in the reproductive units ("flowers").

[3] The female plants produce smaller reproductive units[4] with 8–9(–12)[5] 1.5-2.2mm long bracts enclosing 8-10 carpels.

[2] The type specimen was collected by James Drummond[5][4][6] in Swan River, Western Australia.

[7][8] The description of Hydatella dioica D.A.Cooke was based on a male individual of Trithuria occidentalis Benth.