Tumor-associated macrophages (TAM) of M2 phenotype promote tumor proliferation and are associated with a poor prognosis in patients with glioblastoma. We screened the natural compounds possessing an inhibitory effect on M2 polarization in human monocyte-derived macrophages. Among 130 purified natural compounds examined, corosolic acid significantly inhibited the expression of CD163, one of the phenotype markers of M2 macrophages, and also suppressed the secretion of IL-10, one of the anti-inflammatory cytokines preferentially produced by M2 macrophages, thus suggesting that corosolic acid suppresses M2 polarization of macrophages. Furthermore, corosolic acid inhibited the proliferation of glioblastoma cells, U373 and T98G, and the activation of signal transducer and activator of transcription-3 (STAT3) and nuclear factor-kappa B (NF-κB) in both human macrophages and glioblastoma cells. These results indicate that corosolic acid suppresses the M2 polarization of macrophages and tumor cell proliferation by inhibiting both STAT3 and NF-κB activation. Therefore, corosolic acid might be a potential new tool for tumor prevention and therapy.