Plasmonic nanostructures are of potential in acting as a type of optical agents for cancer photothermal therapy. To effectively function as photothermal therapy agents, plasmonic nanostructures are strongly desired to have good biocompatibility and high photothermal conversion efficiencies. In this study, poly(diallyldimethylammonium chloride)‐coated porous Pt nanoparticles are synthesized for photothermal therapy. The Pt nanoparticles possess broadband near‐infrared light absorption in the range from 650 to 1200 nm, therefore allowing for selecting different laser wavelengths for photothermal therapy. The as‐prepared Pt nanoparticles exhibit remarkable photothermal conversion efficiencies under 809 and 980 nm laser irradiation. In vitro studies indicate that the Pt nanoparticles display good biocompatibility and high cellular uptake efficiencies through an endocytosis pathway. Photothermal heating using 808 nm laser irradiation (>7.0 W cm−2, 3 min) leads to notable cytotoxic effect, and more than 70% of cells are photothermally ablated after 3 min irradiation at 8.4 W cm−2. Furthermore, simultaneous application of photothermal therapy synergistically enhances the cytotoxicity of an anti‐cancer drug doxorubicin. Therefore, the porous Pt nanoparticles have great potential as an attractive photothermal agent for cancer therapy.