Partial filial cannibalism by parental males in fish has been regarded as an adaptive behaviour that compensates for the toll that parental care exacts on their body condition, which is supported by empirical evidence. Here we report that parental males of a nest breeding fish enhance their initial body condition and sustain growth through partial filial cannibalism. Males of the long-snout clingfish (Diademichthys lineatus) use scarcely available empty shells as nests, which limit the number of breeding males. Males continuously breed up to 4 months, during which they hardly leave the nests. Soon after breeding starts, all care-giving males exclusively and frequently consume some of their own eggs. In contrast to previous reports on fish cannibalism, almost all care-giving males grew and enhanced their initial body condition from the early days of care, where males with larger and better-conditioned bodies cannibalised more eggs. Male—male fights for nests were frequently observed: males with larger and more robust bodies won and sometimes took over breeding nests, and it is likely that defeated males could not breed unless they occupy a nest again. These observations indicate that in this fish, partial filial cannibalism produces cannibals of large and robust bodies, and may be advantageous to care-giving males in defending nests against rival males. We suggest that the ability to defend a nest will ensure a longer care-period, and thus, a higher reproductive output. This is the first documented evidence that filial cannibalism enhances the initial body condition of cannibals.