Ligand exchange of CTAB-protected gold nanorods (AuNRs) with disulfide initiator was found to usually cause nanorod aggregation, but such aggregation could be prevented under the steric hindrance of pre-anchored poly(ethylene glycol) thiol. The obtained initiator-modified AuNRs were well-dispersed and could initiate the in situ atom-transfer radical polymerization (ATRP) of 4-vinylpyridine, resulting in poly(4-vinylpyridine)-grafted AuNRs as core-shell structures (AuNR@PVP). These polymer/Au nanocomposites displayed pH-responsive surface plasmon resonance changes because of the protonation and deprotonation of pyridine groups. The coordinative polymer shells allowed these structures to be employed as nanosupports for transition metal ions such as platinum ions, which could be reduced to Pt nanoparticles embedded on the surfaces of the AuNRs. Bimetallic nanostructures of Pt-decorated AuNR@PVP nanocomposites exhibited typical catalytic activity for methanol oxidation.