Shape-anisotropic polymeric colloids having chemically distinct compartments are promising materials, however, introducing site-specific surface functionality to block copolymer (BCP) particles has not yet been actively investigated. The current contribution demonstrates the selective surface functionalization of nanostructured, ellipsoidal polystyrene-b-polybutadiene (PS-b-PB) particle and investigate their effects on the particle shape. Photo-induced thiol-ene click reaction was used as a selective functionalization chemistry for modifying the PB block, which was achieved by controlling the feed ratio of functional thiols to the double bonds in PB. Importantly, the controlled particle elongation was observed as a function of the degree of PB functionalization. Such an increase in the aspect ratio is attributed to the (i) increased incompatibility of the PS and modified PB block and (ii) the reduced surface tension between the particles and surrounding aqueous medium, both of which contributes to the further elongation of ellipsoids. Further tunability of the elongation behavior of ellipsoids was further demonstrated by controlling the particle size and chemical structure of functional thiols, showing the versatility of this approach for controlling the particle shape. Finally, the utility of surface functionality was demonstrated by the facile complexation of fluorescent dye on the modified surface of the particle via favorable interaction, which showed stable fluorescence and colloidal dispersity.