Background <p>The saccus vasculosus is a specialized ependymal organ, found exclusively in osteichthyes and chondrichthyes, and is located on the ventral surface of the diencephalon, posterior to the hypophysis. The structural details of the of saccus vasculosus of <i>Piaractus brachypomus</i> (Characiformes: Serrasalmidae: Colossomatinae) were studied using both light and electron microscopy.</p> Results <p>The saccus vasculosus was a highly vascularized structure, consisting of numerous loculi lined with stratified epithelium that contained characteristic coronet cells and supporting cells. Both cell types rested on a basement membrane, beneath which lay the vascular elements. The coronet cells exhibited distinct basal and apical regions. The spherical nucleus was situated in the basal part, whereas the apical cytoplasm formed a globular extension that protruded into the luminal cavity. The supporting cells were fewer in number and considerably smaller than the coronet cells, with nuclei positioned at the basal region. Unlike the coronet cells, they lacked apical protrusions. Abundant secretory material was observed within the lumen of the loculi, confirming the secretory nature of the saccus vasculosus. In addition, the contact of coronet cells with nerve terminals suggested a sensory function.</p> Conclusions <p>The various cell types of the saccus vasculosus in <i>P. brachypomus</i> were correlated with their possible physiological roles and functional significance.</p>