Combining photon or proton radiotherapy with targeted therapy shows promise for head and neck cancer (HNSCC). The poly (adenosine diphosphate [ADP]-ribose) polymerase-1/2 inhibitor niraparib targets DNA damage repair (DDR). We evaluated the effects of niraparib in combination with photons or protons, and its effects on the relative biological effectiveness (RBE) of protons, in human HNSCC cell lines.