A new approach to synthesize self‐doped colloidal Cu3−xP NCs with controlled size and localized surface plasmon resonance absorption is reported. These Cu3−xP NCs show ultrafast exciton dynamics and huge optical nonlinearities due to plasmonic resonances, which afford the first demonstration of plasmonic Cu3−xP NCs as simple, effective, and solution‐processed nonlinear absorbers for high‐energy Q‐switched fiber laser.