Ovarian cancer is the most lethal gynecologic malignancy. Recently, several molecularly targeted anticancer agents have been developed for ovarian cancer; however, its prognosis remains extremely poor. The development of molecularly targeted therapy, as well as companion diagnostics, is required to improve outcomes for patients with ovarian cancer. In this study, to identify microRNAs (miRNAs) involved in the progression of ovarian cancer we analyzed serum miRNAs in patients with ovarian cancer using miRNA array and quantitative RT-PCR and examined the anticancer properties of miRNA expression in ovarian cancer cells. In patients with ovarian cancer, high amount of miR-135a-3p in serum samples was significantly associated with favorable clinical prognosis. The amount of miR-135a-3p was significantly decreased in patients with ovarian cancer compared with patients with ovarian cysts or normal ovaries. In SKOV-3 and ES-2 human ovarian cancer cells, enhanced expression of miR-135a-3p induced drug sensitivity to cisplatin and paclitaxel and suppressed cell proliferation and xenograft tumor growth. These findings suggest that miR-135a-3p may be considered as a biomarker and a therapeutic agent in ovarian cancer.