An undescribed species of freshwater ostracod belonging to the genus Pseudostrandesia was collected from two pet shops in the Kanto region of central Japan. This species, herein named Pseudostrandesia tenebrarum sp. nov., is similar to four species previously reported from Southeast Asia, but can be distinguished by carapace and appendage features. It is the second species of the genus for which males are known. Of the nine previously described species in the genus, one is exclusively known from Turkey, and the others are found in Southeast Asia and the vicinity, one of which is also recorded in India and east China. There are two scenarios to explain the existence of Pseudostrandesia tenebrarum sp. nov. in pet shops in Japan: it is either native to Japan but has yet to be discovered in its natural habitat, or it is an alien species, perhaps unwittingly imported with plants or fish for the pet trade. We review the likelihood of both scenarios, and conclude that although there is insufficient evidence to be sure, it is potentially an alien species in Japan. The most likely origin is Southeast Asia, as evidenced by its close morphological resemblance to particular Southeast Asian species. Juveniles as well as adults were recovered, indicating that this species is reproducing in the pet trade, supporting the notion that it has invasive potential to areas outside of its natural range. The description and report of this species highlights a possible introduction of an alien species to Japan, and facilitates further monitoring.