The growing incidence of cancer is a problem for modern medicine, since the therapeutic efficacy of applied modalities is still not satisfactory in terms of patients’ survival rates, especially in the case of patients with brain tumors. The destructive influence of chemotherapy and radiotherapy on healthy cells reduces the chances of full recovery. With the development of nanotechnology, new ideas on cancer therapy, including brain tumors, have emerged. Photothermal therapy (PTT) is one of these. It utilizes nanoparticles (NPs) that can convert the light, preferably in the near-infrared (NIR) region, into heat. In this paper, we report the use of nanodiamonds (NDs) conjugated with biomimetic polydopamine (PDA) and indocyanine green (ICG) for glioblastoma cancer PTT therapy. The obtained materials were thoroughly analyzed in terms of their PTT effectiveness, as well as their physicochemical properties. The performed research demonstrated that NDs@PDA@ICG can be successfully applied in the photothermal therapy of glioblastoma for PTT and exhibited high photothermal conversion efficiency η above 40%, which is almost 10 times higher than in case of bare NDs. In regard to our results, our material was found to lead to a better therapeutic outcome and higher eradication of glioblastoma cells, as demonstrated in vitro.