Cesium lead bromide (CsPbBr) has attracted considerable attention as a promising candidate for photovoltaic and optoelectronic applications owing to its relatively high defect tolerance. So far, most studies have focused on single vacancy defects while the effect of higher-order divacancy has been overlooked. Here we uncover the mechanism of formation of the Br divacancy (DV), a popular type of defect that usually develops with time. We predict that DV is formed via merging two isolated Br single vacancies (V) and is stabilized in an apical configuration. Owing to the low formation energy of V, resulting in a high population of vacancy, we predict that DV could be common in CsPbBr, especially for those samples that are highly radiated or exposed to electric field or Br-poor environment. A single V tends to be stabilized as −1 charged defect (V), forming a Pb-Pb dimer, especially in p-doped CsPbBr, and possesses an in-gap defective level at 0.53 eV below the bottom of the conduction band. This Pb-Pb dimer would be the smallest metal Pb cluster in CsPbBr, and likely in other Pb-based perovskites, and is harmful for light emission because of its deep localized defective level. Fortunately, the number of V could be dropped through V = V + e and converted into a begin V being free of in-gap defect level and an unbound Pb-Pb structure. Each DV is stabilized by a redox charge transfer process via 2V → V + V, and also possesses a deep defect level, similar to V. We also demonstrate the ease of forming Cl and I substituent dopants in CsPbBr. Our work can inspire further studies on vacancy clusters in other metal halide perovskites.