First-principles calculations based on the density functional theory were carried out to investigate the magnetic property of phosphorene. It is found that vacancy or external strain alone does not result in magnetism in phosphorene. However, an interplay between vacancy and external strain can lead to magnetism. When either a biaxial strain or a uniaxial strain along the zigzag direction of phosphorene containing P vacancies reaches 4%, the system favors a spin-polarized state with a magnetic moment of ∼1 μB per vacancy site. This is due to spin-polarized p states of under-coordinated P atoms next to the vacancy, which are bonded in the absence of the external strain or when phosphorene is subjected to a low strain.