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Monolayer chromium triiodide (CrI3), as the thinnest ferromagnetic material demonstrated in experiment [Huang et al. Nature 2017, 546, 270], opens up new opportunities for the application of two-dimensional (2D) materials in spintronic nanodevices. Atom-thick 2D materials with switchable electric polarization are now urgently needed for their rarity and important roles in nanoelectronics. Herein, we unveil that surface I vacancies not only enhance the intrinsic ferromagnetism of monolayer CrI3 but also induce switchable electric polarization. I vacancies bring about an out-of-plane polarization without breaking the nonmetallic nature of CrI3. Meanwhile, the induced polarization can be reversed in a moderate energy barrier, arising from the unique porosity of CrI3 that contributes to the switch of I vacancies between top and bottom surfaces. Engineering 2D switchable polarization through surface vacancies is also applicable to many other metal trihalides, which opens up a new and general way toward pursuing low-dimensional multifunctional nanodevices. [ABSTRACT FROM AUTHOR]