Prolactin (PRL)‑like protein J (PLP‑J) is a member of the prolactin family, mainly expressed in the placental decidua tissues of females, and is involved in gestation. To the best of our knowledge, it has not previously been shown to be expressed in males. Preliminary experiments of the present study indicated that PLP‑J is expressed in the testis of male mice and is implicated in the regulation of testicular function. To definitively address whether PLP‑J is expressed in the mouse testis, the expression pattern and cellular localization of PLP‑J in mouse testes during postnatal development were characterized in the current study using molecular and immunological methods. Reverse transcription (RT)‑polymerase chain reaction (PCR) was performed to amplify gene fragments from mouse testis specimens, which yielded sequences matching those of the PLP‑J gene in Genbank. Subsequently, in situ hybridization showed that PLP‑J was localized in interstitial tissue of the mouse testis. Immunofluorescence results indicated that PLP‑J and 3β‑hydroxysteroid dehydrogenase 1 were colocalized in testis Leydig cells, confirming PLP‑J expression in Leydig cells. In addition, PLP‑J gene expression levels were examined at different stages of postnatal mouse development in male testis tissues using quantitative RT‑PCR and western blotting. The results revealed that PLP‑J expression levels were lowest in 18‑day‑old mice and highest in adults aged 4 months. Levels observed in 16‑month‑old individuals were lower than those observed in the 4‑month‑old mice, but remained significantly higher than the levels observed in 18‑day‑old mice. Furthermore, the roles of PLP‑J in the murine testis TM3 Leydig cell line were studied. The results demonstrated that the upregulation of PLP‑J expression in TM3 Leydig cells did not affect testosterone production or the cell cycle. In conclusion, this study demonstrated that PLP‑J, a known member of the PRL family that was previously considered to be expressed solely in females, is also expressed in the testis of males with an age‑dependent expression profile. Nevertheless, the physiological role of PLP‑J in males remains unclear.