Carnitine is essential for lipid metabolism in cells and is known to possess antioxidant properties. Previous reports have suggested that antioxidants are able to induce senescence in glioblastoma cells, consequently, in the present study, we investigated the effect of carnitine on glioblastoma cells. Under conditions of hyponutrition (undernutrition), the proliferation of glioblastoma cells was attenuated and the level of intracellular carnitine was increased. Glioblastoma cell proliferation was also attenuated in cultures that were supplemented with exogenous carnitine, where the induction of senescence was detected by senescence-associated β-gal (SA-β-gal) staining. However, there was no evidence of the induction of apoptosis. These effects were not detected when cells were cultured with carnitine plus an inhibitor of p38 mitogen-activated protein kinase (MAPK). It, therefore, appears that carnitine has antioxidant actions in normal cells but induces senescence, which may be regarded as an opposite phenomenon, in glioblastoma cells. Senescence has been reported in cells exposed to temozolomide, which is a standard drug used for the treatment of glioblastoma. Carnitine could, therefore, represent an attractive alternative therapy for glioblastoma.