Accumulating evidence suggests that microRNA-326 (miR-326) serves as a tumor suppressor in the initiation and progression of several human malignancies. However, the biological function and underlying molecular mechanism of miR-326 in prostatic carcinoma (PCa) remains largely unknown. In the present study, we found that miR-326 expression level was significantly downregulated in both primary PCa and castration-resistant PCa (CRPC) tissue samples as detected by qRT-PCR. Downregulation of miR-326 was closely associated with aggressive progression and poor prognosis of primary PCa patients. Gain- and lose- functional experiments revealed that forced expression of miR-326 significantly inhibited cell proliferation, colony formation, migration and invasion, induced G0/G1 cell cycle arrest, and promoted apoptosis in PCa cells in vitro, whereas, knockdown of miR-326 expression showed the opposite results. Overexpression of miR-326 also suppressed tumor growth in xenografted nude mice in vivo. Moreover, Luciferase reporter, qRT-PCR, and western blot assays identified that the 3'-untranslated region (3'-UTR) of Mucin1 (MUC1) was a direct target region of miR-326. Spearman's correlation analysis also confirmed an inverse relationship between miR-326 and MUC1 expressions in primary PCa tissue samples. In addition, restoration of MUC1 expression effectively abrogated the inhibitory effects of miR-326 on PCa proliferation, invasion and migration through the activation of JNK signaling pathway. Therefore, these data indicated that miR-326 functioned as a tumor suppressor in PCa by negatively regulating MUC1, and that miR-326 might serve as a potential therapeutic candidate for PCa treatment.