BackgroundClock circadian regulator (CLOCK) is a core factor of the mammalian biological clock system in regulating female fertility and ovarian physiology. However, CLOCK's specific function and molecular mechanism in porcine granulosa cells (GCs) remain unclear. In this study, we focused on CLOCK’s effects on GC proliferation.ResultsCLOCK significantly inhibited cell proliferation in porcine GCs. CLOCK decreased the expression of cell cycle-related genes, including CCNB1, CCNE1, and CDK4 at the mRNA and protein levels. CDKN1A levels were upregulated by CLOCK.ASB9is a newly-identified target of CLOCK that inhibits GC proliferation; CLOCK binds to the E-box element in theASB9promoter.ConclusionsThese findings suggest that CLOCK inhibits the proliferation of porcine ovarian GCs by increasingASB9level.Supplementary InformationThe online version contains supplementary material available at 10.1186/s40104-023-00884-7.