Cleavage of cohesins and cyclin-dependent kinase (CDK) inhibition are thought to be sufficient for triggering chromosome segregation. Here we identify an essential requirement for anaphase chromosome movement. We show that, at anaphase onset, the phosphatase Cdc14 and the polo-like kinase Cdc5 are redundantly required to drive spindle elongation. This role of Cdc14 is mediated by the FEAR network, a group of proteins that activates Cdc14 at anaphase onset, and we suggest that Cdc5 facilitates both Cdc14 activation and CDK inhibition. We further identify the kinesin-5 motor protein Cin8 as a key target of Cdc14. Indeed, Cin8 mutants lacking critical CDK phosphorylation sites suppress the requirement for Cdc14 and Cdc5 in anaphase spindle elongation. Our results indicate that cohesin dissolution and CDK inhibition per se are not sufficient to drive sister chromatid segregation but that the motor protein Cin8 must be activated to elongate the spindle.