Updated project metadata. Signal transduction by Small Ubiquitin-like Modifier (SUMO) regulates a myriad of nuclear processes. Here, we report on the role of SUMO in mitosis in human cell lines. Knocking down the SUMO conjugation machinery resulted in a delay in mitosis and defects in mitotic chromosome separation. Searching for relevant SUMOylated proteins in mitosis, we identified the APC/C complex, a master regulator of metaphase to anaphase transition, as a SUMO target. The APC4 subunit is the major SUMO target in the complex, containing SUMO acceptor lysines at positions 772 and 798. SUMOylation increased APC/C ubiquitylation activity towards a subset of its targets, including the newly identified target KIF18B. Intriguingly, SUMOylation of APC/C was regulated by casein kinase driven phosphorylation of serines at positions 777 and 779. Combined, our findings demonstrate the importance of SUMO signal transduction for genome integrity during mitotic progression and reveal a triad of PTMs, cooperating to drive mitosis.