The pseudobranch is a gill-like structure that exhibits great variations in structure and function among fish species, and therefore, it has remained a topic of investigation for a long time. This study was conducted on adult Molly fish (Poecilia sphenops) to investigate the potential functions of their pseudobranch using histological, histochemical, immunohistochemical analysis, and scanning electron microscopy. The pseudobranch of Molly fish was of embedded type. It comprised many rows of parallel lamellae that were fused completely throughout their length by a thin connective tissue. These lamellae consisted of a central blood capillary, surrounded by large secretory pseudobranch cells (PSCs). Immunohistochemical analysis revealed the expression of PSCs for CD3, CD45, iNOS-2, and NF-κB, confirming their role in immunity. Furthermore, T-lymphocytes-positive CD3, leucocytes-positive CD45, and dendritic cells-positive CD-8 and macrophage- positive APG-5 could be distinguished. Moreover, myogenin and TGF-β-positive PSCs were identified, in addition to nests of stem cells- positive SOX-9 were detected. Melanocytes, telocytes, and GFAP-positive astrocytes were also demonstrated. Scanning electron microscopy revealed that the PSCs were covered by microridges, which may increase the surface area for ionic exchange. In conclusion, pseudobranch is a highly specialized structure that may be involved in immune response, ion transport, acid–base balance, as well as cell proliferation and regeneration.