We report an unusual case of unilateral spontaneous lens dislocation in a 52-year-old woman with a posterior polar cataract. We postulate that the increasing lens size secondary to nuclear sclerosis may have exerted pressure on a thinned posterior capsule, causing the capsule to rupture spontaneously. The unusual weak posterior capsule in posterior polar cataract is a well-recognized risk factor in phacoemulsification surgery. We believe this is the first report case of spontaneous lens dislocation secondary to posterior capsule rupture in posterior polar cataract.