Cucumerunio websteri is a species of freshwater mussel, an aquatic bivalve mollusc in the family Hyriidae.
The specific name websteri is in honor of Reverend William Henry Webster (died 1931) of Wauiku, New Zealand, who have sent specimens to the National Museum of Natural History. Cucumerunio websteri then was described under the name Diplodon websteri by American malacologist Charles Torrey Simpson in 1902.
Simpson's original text (the type description) reads as follows:
The length of the shell is 62–81 mm. The height of the shell is 32–39 mm. The width of the shell is 14–20 mm.
It lives in the North Island, New Zealand. The type locality is New Zealand, but the exact type locality is unknown.
It inhabits lakes and streams.
