Within the past few years, atomically thin black phosphorus (BP) has been demonstrated as a fascinating new 2D material that is promising for novel nanoelectronic and nanophotonic applications, due to its many unique properties such as a direct and widely tunable bandgap, high carrier mobility and remarkable intrinsic in-plane anisotropy. However, its important extreme nonlinear behavior and the ultrafast dynamics of carriers under strong-field excitation have yet to be revealed. Herein, we report nonperturbative high harmonic generation (HHG) in monolayer BP by first-principles simulations. We show that BP exhibits extraordinary HHG properties, with clear advantages over three major types of 2D materials under intensive study, i.e., semimetallic graphene, semiconducting MoS2, and insulating hexagonal boron nitride, in terms of HHG cutoff energy and spectral intensity. This study advances the scope of current research activities on BP into a new regime, suggesting its promising future in the applications of extreme-ultraviolet and attosecond nanophotonics and also opening doors to investigate the strong-field and ultrafast carrier dynamics of this emerging material.