Carboxysomes are bacterial microcompartments that function as the centerpiece of the bacterial CO2-concentrating mechanism, feeding high concentrations of CO2 to the enzyme Rubisco for fixation. The carboxysome self-assembles from thousands of individual proteins into icosahedral-like particles with a dense enzyme cargo encapsulated within a proteinaceous shell. In the case of the α-carboxysome, there is little molecular insight into protein-protein interactions which drive the assembly process. Here we show that the N-terminus of CsoS2, an intrinsically disordered protein found in the α-carboxysome, possesses a repeated peptide sequence that binds Rubisco. X-ray structural analysis of the peptide bound to Rubisco reveals a series of conserved electrostatic interactions that are only made with properly assembled hexadecameric Rubisco. Although biophysical measurements indicate this single interaction is weak, its implicit multivalency induces high-affinity binding through avidity. Taken together, our results indicate CsoS2 acts as an interaction hub to condense Rubisco and enable efficient α-carboxysome formation.