Transition metal nitrides are robust alternatives to noble metals in plasmonics, offering strong NIR absorption, high melting points, and chemical stability. Hafnium nitride (HfN) nanoparticles are especially promising owing to the high atomic number of hafnium, which could provide X-ray-based theranostic functionalities in addition to the prominent plasmonic properties. However, their biomedical potential has remained unexplored due to difficulties in synthesis of pure water-dispersible nanostructures. Here, we use femtosecond laser ablation in acetone to produce HfN-based nanoparticles exhibiting a red-shifted plasmon resonance spanning both the NIR-I and NIR-II windows. We then present the first in vivo assessment of the biocompatibility, photothermal performance, and radiosensitizing capacity of HfN-based nanomaterials. We show that PEGylated HfN nanoparticles exhibit negligible cytotoxicity across three cancer cell lines in vitro and no significant long-term adverse effects in healthy mice following intravenous administration. NIR-I photothermal therapy of 4T1 tumor-bearing mice after systemic nanoparticle administration leads to a 2.4-fold suppression of tumor growth and a significant extension of survival. The multimodal therapeutic potential of HfN is demonstrated by the enhanced efficacy of X-ray radiotherapy after local administration of nanoparticles. Laser ablated engineering of hafnium nitrides establishes this nanomaterial as a promising candidate for multimodal optical and radiosensitizing theranostics.