The majority of breast cancers are primarily hormone-sensitive and can be managed by endocrine therapy, although therapy-resistant or hormone-refractory cancers need alternative treatments. Recently, increasing attention is being paid to RNA-binding proteins (RBP) in cancer pathophysiology. The precise role of RBP in breast cancer, however, remains to be clarified. We herein show that an RBP non-POU domain-containing octamer binding (NONO) plays a critical role in the pathophysiology of breast cancers regardless of their hormone dependency. Clinicopathological and immunohistochemical study of 127 breast cancer cases showed that NONO is a significant independent prognostic factor for breast cancer patients. Notably, siRNA-mediated NONO knockdown substantially repressed the proliferation of both hormone-sensitive MCF-7 and hormone-refractory MB-MDA-231 breast cancer cells. Integrative analysis combined with expression microarray and RIP-sequencing (RNA immunoprecipitation-sequencing) showed that NONO post-transcriptionally regulates the expression of cell proliferation-related genes by binding to their mRNAs, as exemplified by S-phase-associated kinase 2 and E2F transcription factor 8. Overall, these results suggest that NONO is a key regulator for breast cancer proliferation through the pre-mRNA splicing of cell proliferation-related genes and could be a potential new diagnostic and therapeutic target for advanced disease.