Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant neurologic disorder, whose symptoms include cerebellar ataxia and epilepsy. The disease is caused by ATTCT expansion in the ATXN10 gene, which encodes the Ataxin-10 protein. Here we identified polo-like kinase 1 (Plk1) as one of Ataxin-10's binding partners. We show that epitope-tagged Ataxin-10 and Plk1 coimmunoprecipitate, and Plk1 phosphorylates Ataxin-10 at S77 and T82 in vitro. Knockdown of ATXN10 with siRNA in HeLa cells results in cytokinesis defects-multinucleation, which are rescued by wild-type Ataxin-10, but not the phosphor-deficient 2A mutant. Phosphorylation-specific antibodies towards pS77 detect specific signals at the midbody. Like the knockdown, overexpression of the 2A mutant generates multinucleated cells and the 2A mutant shows decreased interaction with the Plk1 polo-box domain. In addition, we found that Ataxin-10 is ubiquitinated, and is subject to proteasome-dependent degradation, which is delayed in the 2A mutant. We propose a model in which Plk1 phosphorylation of Ataxin-10 influences its degradation and cytokinesis, which may provide mechanistic insight to SCA10's pathogenesis.