In the neuronal system, polysialic acid (polySia) is known to be involved in several cellular processes such as the modulation of cell–cell interactions. This highly negatively-charged sugar moiety is mainly present as a post-translational modification of the neural cell adhesion molecule (NCAM). More than 20 years ago, differently glycosylated forms of NCAM were detected in the ovaries. However, the exact isoform of NCAM, as well as its biological function, remained unknown. Our analysis revealed that granulosa cells of feline tertiary follicles express the polysialylated form of NCAM-140. Unexpectedly, polySia was only expressed in the granulosa layers of atretic follicles and not of healthy follicles. By contrast, only the un-polysialylated form of NCAM was present on the membrane of granulosa cells of healthy follicles. To study a possible cellular function of polySia in feline follicles, a primary granulosa cell culture model was used. Interestingly, loss of polySia leads to a significant inhibition of apoptosis, demonstrating that polySia is involved during atretic processes in granulosa cells. Thus, polySia might not only directly influence regeneration processes as shown, for example, in the neuronal system, but also apoptosis.