The anaphase-promoting complex/cyclosome (APC/C) controls the onset of anaphase by targeting securin for destruction. We report here the identification and characterization of a substrate of APC/C, RCS1, as a mitotic regulator that controls the metaphase-to-anaphase transition. We showed that the levels of RCS1 fluctuate in the cell cycle, peaking in mitosis and dropping drastically as cells exit into G(1). Indeed, RCS1 is efficiently ubiquitinated by APC/C in vitro and degraded during mitotic exit in a Cdh1-dependent manner in vivo. APC/C recognizes a unique D-box at the N terminus of RCS1, as mutations of this D-box abolished ubiquitination in vitro and stabilized the mutant protein in vivo. RCS1 controls the timing of the anaphase onset, because the loss of RCS1 resulted in a faster progression from the metaphase to anaphase and accelerated degradation of securin and cyclin B. Biochemically, mitotic RCS1 associates with the NuRD chromatin-remodeling complex, and this RCS1 complex is likely involved in regulating gene expression or chromatin structure, which in turn may control anaphase onset. Our study uncovers a complex regulatory network for the metaphase-to-anaphase transition.