Heparan sulfate proteoglycans are known to assist HIV-1 entry into host cells, mediated by the viral envelope glycoprotein gp120. We aimed to determine the general structural features of glycosaminoglycans that enable their binding to gp120, by surface plasmon resonance. Binding was found to be dependent on sequence type, size and sulfation patterns. HIV-1 gp120 prefers heparin and heparan sulfate (with at least 16 monomers in length) over chondroitin and dermatan. Sulfate groups were essential to promote this interaction. These results advance the understanding of the molecular-level requirements for virus attachment and cell entry.