Unlike in budding yeast, sister chromatid cohesion in vertebrate cells is resolved in two steps: cohesin complexes are removed from sister chromatid arms during prophase via phosphorylation, whereas centromeric cohesins are removed at anaphase by Separase. Phosphorylation of cohesin subunit SA2 by polo-like kinase 1 (Plk1) is required for the removal of cohesins at prophase, but how Plk1 is recruited to phosphorylate SA2 during prophase is currently not known. Here we report that Sororin, a cohesin-interacting protein essential for sister chromatid cohesion, plays a novel role in the resolution of sister chromatid arms by direct interaction with Plk1. We identified an evolutionarily conserved motif (ST(159)P) on Sororin, which was phosphorylated by Cdk1/cyclin B and bound to the polo box domain of Plk1. Mutating Thr(159) into alanine prevented the interaction of Plk1 and Sororin and inhibited the resolution of chromosomal arm cohesion. We propose that Sororin is phosphorylated by Cdk1/cyclin B at prophase and acts as a docking protein to bring Plk1 into proximity with SA2, resulting in the phosphorylation of SA2 and the removal of cohesin complexes from chromosomal arms.