2D materials such as MXenes are recognised for their therapeutic potential in biomedicine. However, morphology of nanomaterials determines their interactions with biological systems, and uneven shape of MXenes impose critical limitations on their clinical applicability. Here, we introduce spherical MXenes as a novel class of biocompatible nanomaterials, and demonstrate applicability of Ti3C2 nanospheres for photothermal therapy (PTT) of breast cancer. The Ti3C2 nanospheres were prepared by femtosecond laser fragmentation of MXene powder and retained the crystallinity and internal optical properties of the parent material. The nanoparticles have characteristic light absorption both in the NIR-I and NIR-II windows, with a superior photothermal conversion efficiency of 68% and 63% under 808-nm and 1064-nm laser irradiation, respectively. The Ti3C2 nanospheres did not induce photodynamic effects and demonstrated negligible toxicity in vitro in three different cell lines and in vivo in healthy mice. Under laser irradiation, the Ti3C2 nanospheres exhibit potent photothermal cytotoxicity as shown in cell monolayers, spheroids and a 4T1 murine tumor model. When employed for PTT, they significantly inhibit tumor growth and 1.4-fold prolong median survival of animals. These findings demonstrate biocompatibility and therapeutic potential of spherical MXenes for cancer management.