During cell division, the onset of mitosis is granted by activation of cyclin B-dependent kinase 1 (Cdk1), master mitotic kinase, coordinated with inactivation of Cdk1-counteracting phosphatases. PP2A-B55, a major of these phosphatases, is inhibited in mitosis by Arpp-19 and Ensa, two very similar proteins, once phosphorylated by the Cdk1-stimulated kinase Greatwall (Gwl). We show here that Arpp-19 is also phosphorylated in a Cdk1-dependent manner at serine 23, a site missing in mammalian Ensa, in mitotic human cells and dephosphorylated at this site during mitosis exit. Moreover, we found that this phosphorylation control grants chromosome stability since substituting endogenous Arpp-19 with a S23-Arpp-19 phosphorylation-resistant mutant increased the frequency of chromosome segregation errors and accelerated the timing of mitosis exit. Conversely, substitution with a S23-Arpp-19 phosphorylation-mimicking mutant delayed mitosis exit. S23-Arpp-19 dephosphorylation resisted to the potent PP2A inhibitor Okadaic Acid but required the phosphatase Fcp1. Our data unveil a phosphorylation switch that grants timely mitosis exit and chromosome stability.