Poly(4-vinylpyridine) (P4VP), a pH-sensitive polymer, was employed to non-covalently functionalize graphene oxides (GOs) through π–π stacking interactions. Citrate-capped Au nanoparticles (Au NPs) were then immobilized onto the P4VP-grafted GO, yielding pH-responsive Au NPs–GO nanocomposites. Such Au NPs–GO nanocomposites displayed pH-responsive localized surface plasmon resonance (LSPR) changes and assembly behavior due to protonation and deprotonation of the pyridine units of P4VP molecules. The catalytic activity of the Au NPs–GO nanocomposites was evaluated using the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction, and demonstrated significantly high catalytic activity and stability at low pH (2.0), which was 4-fold higher than that at high pH (5.0). Furthermore, our results indicated that the Au NPs–GO nanocomposites induced active adsorption and selective SERS detection of negatively charged analytes under low pH conditions, and are therefore highly attractive for real-world chemical detection.