During mitosis, properly aligned chromosomes stabilise microtubule ends with the help of kinetochores to ensure timely segregation of chromosomes. Microtubule-binding components of the human outer kinetochore, such as Ndc80 and Ska complexes, are present in multiple copies and together bind several microtubule ends, creating a highly multivalent binding interface. Whereas Ndc80:Ndc80 and Ndc80:microtubule binding is crucial for interface stability, Ndc80 alone in absence of Ska is unable to support stable kinetochore-attachments. Using cryoET, we demonstrate that oligomeric Ndc80:Ska assemblies stabilise microtubule ends against shortening by strengthening lateral contacts between tubulin protofilaments at microtubule plus-ends. We further identify a point mutation within the SKA1 microtubule-binding domain that does not affect microtubule-binding of individual Ska molecules, but does abolish Ska:Ska interactions. Finally, we report that oligomerisation of Ska, in a cooperative fashion together with the Ndc80, is necessary to maintain stable microtubule attachments both in vivo and in vitro. ### Competing Interest Statement The authors have declared no competing interest. BBSRC, BB/X014975/1, BB/W019698/1 Wellcome Trust, https://ror.org/029chgv08, 308895/Z/23/Z