Terf/TRIM17 is a tripartite motif protein that has been originally isolated from testis. Terf has been characterized to exhibit an E3 ubiquitin ligase activity and to undergo self-ubiquitination. The cellular function of terf and its substrates, however, remain elusive. In the present study, we performed a yeast two-hybrid screening assay using terf as bait and identified a positive clone coding for ZW10 interacting protein (ZWINT), a known component of the kinetochore complex required for the mitotic spindle checkpoint. Immunoprecipitation and western blot analyses showed that terf interacted with ZWINT and that overexpression of terf caused down-regulation of protein levels of ZWINT in mammalian cells. In addition, the coiled-coil domain of terf was required for the interaction with ZWINT. In a cell growth assay, stable transfection with terf decreased proliferation of MCF7 breast cancer cells. In contrast, the growth rate of MCF7 cells was increased by stable expression of ZWINT. Specific siRNAs targeting terf and ZWINT dampened these negative and positive effects of terf and ZWINT on cell proliferation, respectively. These results suggest that the E3 ubiquitin ligase terf causes protein degradation of ZWINT and negatively regulates cell proliferation.