Single-atom　catalysts　(SACs)　have　exhibited　extraordinary　catalytic　performance　due　to　the　utmost　atom　utilization　efficiency　and　unique　electronic　statesviametal-support　interactions.　Rationally　designing　SACs　at　the　atomic　level　by　structural　engineering　is　desirable　for　an　efficient　hydrogen　evolution　reaction　(HER).　Herein,　single-atom　Pt　was　anchored　on　two-dimensional　(2D)　Co(OH)(2)nanosheets　growing　on　Ag　nanowires　(Pt-SA-Co(OH)(2)@Ag　NW)　to　construct　a　seamlessly　conductive　network　hybrid　catalystviaelectrochemical　phase　transformation　from　metallic　Co@Ag　NW.　The　Ag　NW　network　provides　a　continuous　electron　transport　pathway　through　metal　active　sites,　contributing　to　an　extremely　low　charge　transfer　resistance　(R-ct,　0.7　omega),　and　the　hierarchical　nanostructure　has　a　large　electrochemical　surface　area　for　high　atom　utilization　efficiency　and　rich　mass　transport　pathways　for　hydrogen　generation　and　release.　Moreover,　Co(OH)(2)tailored　Pt　sites　induce　a　local　tip-enhancement　electric　field　region　around　the　Pt　site　and　more　d　contribution　for　boosting　H　adsorption　and　H2O　adsorption.　Thus,　the　synthesized　Pt-SA-Co(OH)(2)@Ag　NW　catalyst　shows　outstanding　HER　activity　with　only　29　mV　overpotential　in　1.0　M　KOH　at　10　mA　cm(-2)and　22.5-fold　higher　mass　activity　than　the　commercial　Pt/C　catalyst.