c-Jun N-terminal kinase (JNK) plays a vital role in malignant transformation of different cancers, and JNK is highly activated in basal-like triple-negative breast cancer (TNBC). However, the roles of JNK in regulating cancer stem-like cell (CSC) phenotype and tumorigenesis in TNBC are not well defined. JNK is known to mediate many cellular events via activating c-Jun. Here, we found that JNK regulated c-Jun activation in TNBC cells and that JNK activation correlated with c-Jun activation in TNBC tumors. Furthermore, the expression level of c-Jun was significantly higher in TNBC tumors than in non-TNBC tumors, and high c-Jun mRNA level was associated with shorter disease-free survival of patients with TNBC. Thus, we hypothesized that the JNK/c-Jun signaling pathway contributes to TNBC tumorigenesis. We found that knockdown of JNK1 or JNK2 or treatment with JNK-IN-8, an adenosine triphosphate-competitive irreversible pan-JNK inhibitor, significantly reduced cell proliferation, the ALDH1+ and CD44+/CD24- CSC subpopulations, and mammosphere formation, indicating that JNK promotes CSC self-renewal and maintenance in TNBC. We further demonstrated that both JNK1 and JNK2 regulated Notch1 transcription via activation of c-Jun and that the JNK/c-Jun signaling pathway promoted CSC phenotype through Notch1 signaling in TNBC. In a TNBC xenograft mouse model, JNK-IN-8 significantly suppressed tumor growth in a dose-dependent manner by inhibiting acquisition of the CSC phenotype. Taken together, our data demonstrate that JNK regulates TNBC tumorigenesis by promoting CSC phenotype through Notch1 signaling via activation of c-Jun and indicate that JNK/c-Jun/Notch1 signaling is a potential therapeutic target for TNBC.