The invasive Pallas’s squirrel (Callosciurus erythraeus) is now established in Japan, and it is abundantly fertile, which results in a constant increase in the population and distribution. The reproductive characteristic of this invasive squirrel is a continuous breeding, although native squirrels demonstrate seasonal breeding. The reproductive seasonality of this invasive squirrel seems to be less affected by seasonal changes in food availability, which regulate reproductive seasonality in native species. In the present study, we postulated that testis size index, an indicator of male reproductive function, does not differ between seasons with high (June-November) and low (December-May) food availability, and that it positively correlates with the body condition index throughout the year due to energy allocation to the testis. We therefore assessed seasonal changes in testis size index in euthanized 284 Pallas’s squirrels, and seasonal associations between testis size and body condition indexes. Testis size index did not differ between seasons. Furthermore, testis size and body condition indexes were positively and significantly associated. Our findings suggest that whereas male reproductive effort in this invasive squirrel consumes energy like native species, testis function maintains stable in a whole year. This could be a characteristic of continuous breeding that might be associated with the robust reproductive activity of this squirrel.