A stochastic gravitational-wave background can inevitably generate second-order scalar perturbations, which may collapse into primordial black holes (PBHs) above a calculable GW amplitude threshold. In this talk, I will present this new GW-induced PBH formation mechanism and apply it to primordial GWs, first-order phase transitions, and domain-wall annihilation. I will discuss the implications for the NANOGrav/EPTA nanohertz signal, including the predicted PBH mass range and dark-matter abundance, and highlight the complementarity between current and future GW observatories and PBH searches.