Elevated level of procathepsin D (pCD), a zymogen of lysosomal aspartic proteinase cathepsin D, is associated with highly invasive neoplasms that include breast cancer. Independent studies have established that secreted pCD functions as a growth factor acting both in an autocrine and paracrine manner. Therefore, to explore whether pCD can be employed as a therapeutic target, the present study evaluates the impact of pCD knockdown using RNA interference technology. Of the three siRNA oligos tested, siRNA-3 exhibited a 90% inhibitory effect on pCD gene expression. Stable attenuation of pCD in breast cancer cells MDA-MB-231 was achieved by using a plasmid vector-based shRNA system. Pronounced suppression of pCD expression was accompanied by a significant reduction in invasion and proliferation of MDA-MB-231 cells stably transfected with functional shRNA. Importantly, in the athymic nude mice model, downregulation of pCD in breast cancer cells significantly reduced their metastatic potential. In addition, we observed a reduction in Cdc42 and NFkappaB2 expression in MDA-MB-231 cells with decreased pCD expression. When combined, our in vitro and in vivo experiments demonstrate that targeting pCD through RNAi technology represents a potential therapeutic tool for developing a therapy against breast cancer.