We demonstrate that layered hydrogel poly(methacrylic acid) capsules (PMAA), produced from hydrogen-bonded (PMAA/poly-N-vinylpyrrolidone) (PMAA/PVPON) multilayer precursors through cross-linking with ethylenediamine (EDA), can facilitate in situ synthesis of gold nanoparticles within hydrogel walls under ambient conditions. The necessary amine groups are available within the (PMAA) shells due to one-end attached cross-linker molecules. We also show that the nanoparticle size can be controlled through changing the pH-dependent balance of amine/ammonium groups in the ionic cross-links within the shells. Importantly, the pH-responsive properties of the ultrathin hydrogel shells are preserved after gold nanoparticle synthesis within the capsule walls. The reported in situ synthesis of gold nanoparticles within the ultrathin and pH-responsive shells can find a potential use for straightforward and facile fabrication of the hybrid organic-gold nanomaterials for biochemical sensing and delivery applications. © 2009 American Chemical Society.