The faithful segregation of chromosomes during cell division is essential for avoiding DNA damage and aneuploidy, both of which may lead to cancer. One prerequisite for chromosome segregation is the establishment of stable end-on kinetochore–microtubule attachments, which requires the kinase Plk1. Now, Ulrike Gruneberg and colleagues (Geraghty et al., 2021) investigate the role of Plk1 in regulating the kinetochore and mitotic spindle protein astrin. They find that Plk1 and astrin colocalise at attached kinetochores and that astrin is unable to bind to kinetochores in the absence of Plk1. Plk1 phosphorylates multiple residues within the astrin N-terminus, with astrin double-phosphorylated at S157 and S159 localising to attached kinetochores in a Plk1-dependent manner. Mutation of the Plk1 interaction site within astrin prevents its phosphorylation at S157 and S159 and reduces its association with attached kinetochores. The authors show that, in cells with lower kinetochore-bound astrin (expressing astrin with mutation of its Plk1-binding site or residues phosphorylated by Plk1), kinetochore–microtubule attachment is less robust to cold treatment. Moreover, upon prolonged metaphase arrest, these cells cannot maintain the metaphase plate for as long as cells expressing wild-type astrin can. Collectively, these findings highlight a new role for Plk1 in recruiting astrin to kinetochores, which then stabilises bi-oriented kinetochore–microtubule attachments.