The precise control of plasmonic nanostructures and their use for less invasive apoptotic pathway-based therapeutics are important but challenging. Here, we introduce a highly controlled synthetic strategy for plasmonic core-petal nanoparticles (CPNs) with massively branched and plasmonically coupled nanostructures. The formation of CPNs was facilitated by the gold chloride-induced oxidative disassembly and rupture of the polydopamine corona around Au nanoparticles and subsequent growth of Au nanopetals. We show that CPNs can act as multifunctional nanoprobes that induce dual photodynamic and photothermal therapeutic effects without a need for organic photosensitizers, coupled with the generation of reactive oxygen species (ROS), and allow for imaging and analyzing cells. Near-infrared laser-activated CPNs can optically monitor and efficiently kill cancer cells via apoptotic pathway by dual phototherapeutic effects and ROS-mediated oxidative intracellular damage with a relatively mild increase in temperature, low laser power, and short laser exposure time.