There is great potential for photodynamic therapy (PDT)-enhanced photothermal therapy (PTT) to be used for tumor therapy, especially for the single material-mediated process that could greatly simplify the experimental arrangements. This study presents a new cancer phototherapeutic agent consisting of low-work-function lanthanum hexaboride particles, which are excellent light absorbers in the near-infrared (NIR) region. The photothermal effect and reactive oxygen species production were realized by LaB6 under NIR light irradiation. Theoretical calculations based on density functional theory confirmed that the strong NIR light absorption by LaB6 was attributed to the local plasmonic resonance effect and the excellent photodynamic effect derived from the low work function. In vivo treatment of HepG2 tumor-bearing mice revealed that LaB6-mediated phototherapy resulted in excellent tumor inhibitory effects, and no adverse effects on mice were observed. These results indicate that LaB6 is a promising phototherapeutic agent for cancer synergetic phototherapy.