Ovarian cysts are linked to hormone imbalances and altered gene expressions, but the connection between cysts and ion channel expression is understudied. This study explored the role of TWIK-related acid-sensitive K+ (TASK) channels in bovine ovarian cyst formation. The ovarian follicles were split into small (5 to 10 mm in diameter) and large (> 25 mm in diameter) groups. Among the measured K+, Na+, and Cl- concentrations in follicular fluid (FF) obtained from small-sized follicles (SF) and large-sized follicles (LF), the K+ concentration was significantly lower in LFFF. Quantitative PCR, western blot, and immunocytochemistry data revealed that TASK-3 expression levels significantly decreased by approximately 50% in both LF and granulosa cells obtained from LF (LFGCs) compared to the corresponding controls. The TASK-3 protein was localized to the plasma membrane of GCs. The diameter of LFGCs was larger than that of SFGC. The cell swelling response to exposure to a hypotonic solution (200 mOsm/L) was highly reduced in TASK-3-overexpressing cells compared to vector-transfected cells. TASK-3-knockdown cells showed arrested growth. Senescence markers were detected in LFGCs and TASK-3-knockdown cells. These findings suggest that reduced TASK-3 expression in LF is associated with the inhibition of GC growth, leading to senescence and cyst formation.