Injectable thermosensitive hydrogels have been widely investigated for drug delivery systems. Chitosan (CH) is one of the most abundant natural polymers, and its biocompatibility and biodegradability make it a favorable polymer for thermosensitive hydrogel formation. The addition of nanoparticles can improve its drug release behavior, remote actuation capability, and biological interactions. Carbon nanotubes (CNTs) have been studied for the use in drug delivery systems, and they can act as drug delivery vehicles to improve the delivery of different types of therapeutic agents. In this work, carbon nanotubes were incorporated into a thermosensitive and injectable hydrogel formed by chitosan and β-glycerophosphate (β-GP) (CH-β-GP-CNTs). The hybrid hydrogels loaded with methotrexate (MTX) were liquid at room temperature and became a solidified gel at body temperature. A number of tests including scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction were utilized to characterize the MTX-loaded CH-β-GP-CNT hybrid hydrogels. The cell viability (alamarBlue) assay showed that hydrogels containing CNT (0.1%) were not toxic to the 3T3 cells. In vitro MTX release study revealed that CNT-containing hydrogels (with 0.1% CNT) demonstrated a decreased MTX releasing rate compared with control hydrogels without CNT. The cultured MCF-7 breast cancer cells were used to evaluate the efficacy of CH-β-GP-CNT hybrid hydrogels delivering MTX on the control of tumor cell growth. Results demonstrated that CNT (0.1%) in the hydrogel enhanced the MTX antitumor function. Our study indicates that a thermosensitive CH-β-GP-CNT hybrid hydrogel can be used as a potential breast cancer therapy system for controlled delivery of MTX.