Malignant pleural mesothelioma (MPM) is an aggressive tumor with an unfavorable prognosis. The standard therapeutic approaches are limited to surgery, chemotherapy, and radiotherapy. Because the consequent clinical outcome is often unsatisfactory, a different approach in MPM treatment is required. S100A11, a Ca2+-binding small protein with two EF-hands, is frequently upregulated in various human cancers. Interestingly, it has been found that intracellular and extracellular S100A11 have different functions in cell viability. In this study, we focused on the impact of extracellular S100A11 in MPM and explored the therapeutic potential of an S100A11-targeting strategy. We examined the secretion level of S100A11 in various kinds of cell lines by enzyme-linked immunosorbent assay. Among them, six out of seven MPM cell lines actively secreted S100A11, whereas normal mesothelial cell lines did not secrete it. To investigate the role of secreted S100A11 in MPM, we inhibited its function by neutralizing S100A11 with an anti-S100A11 antibody. Interestingly, the antibody significantly inhibited the proliferation of S100A11-secreting MPM cells in vitro and in vivo. Microarray analysis revealed that several pathways including genes involved in cell proliferation were negatively enriched in the antibody-treated cell lines. In addition, we examined the secretion level of S100A11 in various types of pleural effusions. We found that the secretion of S100A11 was significantly higher in MPM pleural effusions, compared to others, suggesting the possibility for the use of S100A11 as a biomarker. In conclusion, our results indicate that extracellular S100A11 plays important roles in MPM and may be a therapeutic target in S100A11-secreting MPM.