The molecular circadian oscillators in mammals rely on a negative feedback loop involving the transcriptional repressors PER1, PER2, CRY1 and CRY2. Here we report the identification of CAVIN-3 as a novel, cytoplasmic PER2-interacting protein. We found that cellular CAVIN-3 levels directly influenced circadian clock properties. Loss- and gain-of-function of CAVIN-3 thus shortened and lengthened, respectively, the circadian period in fibroblasts. CAVIN-3 had a marked effect on the abundance of PER and CRY proteins, which were increased and decreased upon CAVIN-3 depletion and overexpression, respectively, and probably on the strength of the PER:CRY interaction, as suggested by mammalian two-hybrid assays. A CAVIN-3 variant carrying a mutated binding site for the previously reported interacting protein, Protein Kinase C (PKC) δ, failed to alter circadian period length. The role of CAVIN-3 in the clockwork may thus at least in part be mediated by its interaction with yet uncharacterized protein kinases.