Vitamin K2 [menaquinone-4 (MK-4)] has been reported to induce apoptosis in hepatocellular carcinoma, leukemia, and MDS cell lines. The effects of MK-4 on the development of arthritis have never been addressed so far. In this study, we investigated the effect of MK-4 upon the proliferation of rheumatoid synovial cells and the development of arthritis in collagen-induced arthritis (CIA). We analyzed the effect of MK-4 on the proliferation of fibroblast-like synoviocytes (FLSs) using the 3-(4,5-demethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The proapoptotic effect of MK-4 upon FLS was investigated with annexin V staining and DNA fragmentation and caspase 3/7 assays. Moreover, we analyzed the effect of MK-4 on the development of CIA in female dark agouti rats. Our results indicated that MK-4 inhibited the proliferation of FLS and the development of CIA in a dose-dependent manner. We concluded that MK-4 may represent a new agent for the treatment of RA in the setting of combination therapy with other disease-modifying antirheumatic drugs.