SEPT9_v1, the largest transcript of the septin gene family member, SEPT9, encodes a septin isoform implicated in the tumorigenic transformation of mammary epithelial cells. High levels of SEPT9_v1 expression also have been observed in both breast cancer cell lines, primary breast cancers as well as other solid tumor malignancies. We found a novel interaction between SEPT9_v1 and the c-Jun-N-terminal kinase (JNK), a mitogen-activated protein kinase important in cellular stress responses, cell proliferation, and cell survival. We found that up-regulation of SEPT9_v1 stabilizes JNK by delaying its degradation, thereby activating the JNK transcriptome. C-jun kinase assays in mammary epithelial cells expressing SEPT9_v1, compared to controls, exhibited increased JNK/c-Jun transcriptional activity. This increase was associated with increased levels of cyclin D1, a critical component of the proliferative response required for progression through G(1) of the cell cycle in many cell types. These findings demonstrate the first link between a septin protein and the JNK signaling pathway. Importantly, it suggests a novel functional role of SEPT9_v1 in driving cellular proliferation of mammary epithelial cells, a hallmark feature of oncogenesis that is directly relevant to breast cancer.