Branched　plasmonic　nanostructures　have　been　found　to　exhibit　strong　enhancement　of　the　electromagnetic　field　surrounding　their　multi-branched　petals.　This　feature　endows　them　with　improved　performance　in　catalysis,　surface-enhanced　Raman　scattering,　photoacoustic　imaging,　and　photothermal　therapy.　Albeit　several　synthesis　techniques　have　been　developed,　the　precisely　controlled　growth　of　highly　branched　nanostructures　with　a　one-pot　surfactant-free　procedure　is　still　challenging.　Herein,　we　present　a　simple　seedless　route　to　synthesize　gold　nanostars　(AuNSs)　using　a　natural　plant　polyphenol,　gallic　acid　(GA),　as　a　reducing　and　stabilizing　agent.　The　size　and　shape　of　AuNSs　can　be　tuned　by　simply　adjusting　the　amount　of　added　GA.　Under　the　optimum　condition,　the　as-prepared　AuNSs　with　diameters　about　100　nm　exhibit　strong　near-infrared　absorption,　good　photothermal　efficiency,　and　high　biocompatibility.　We　demonstrate　that　AuNSs　can　be　utilized　for　simultaneous　photoacoustic　imaging　and　photothermal　therapy　in　living　cancer　cells.　This　study　highlights　facile　synthesized　AuNSs　could　serve　as　a　promising　platform　for　cancer　diagnosis　and　therapy.