The development of quantum technologies on nanophotonic platforms has seen momentous progress in the past decade. Despite that, a demonstration of time-frequency entanglement over a broad spectral width is still lacking. Here we present an efficient source of ultrabroadband entangled photon pairs on a periodically poled lithium niobate nanophotonic waveguide. Employing dispersion engineering, we demonstrate a record-high 100 THz () generation bandwidth with a high efficiency of and excellent noise performance with the coincidence-to-accidental ratio exceeding . We also measure strong time-frequency entanglement with over 98% two-photon interference visibility.