Polydopamine nanoparticles (PDA NPs) are an appealing biomimetic photothermal agent for photothermal antibacterial treatment because of their long-term safety, excellent photostability, accessible manufacturing, and good biodegradability. However, the low photothermal conversion efficiency (PCE) of PDA NPs requires high-power and long-term near-infrared light irradiation, which severely restricts their practical application. In this work, PDA@Cu NPs were fabricated by growing Cu NPs in situ on the surface of PDA and then introduced into a polyelectrolyte hydrogel precursor (cationic polyethyleneimine/anionic pectin, named as CPAP). The formulated photothermal platform possessed a high PCE (55.4%), almost twice as much as pure PDA NPs (30.8%). Moreover, the designed CPAP/PDA@Cu captured and killed some bacteria by electrostatic adsorption, which helped enhance the antibacterial performance. As expected, the formed CPAP/PDA@Cu that combined the advantageous features of PDA@Cu NPs (high PCE) and CPAP matrix (inherent antibacterial activity and preventing NPs aggregation) can efficiently kill bacteria both in vitro and in vivo under the help of near-infrared laser irradiation. Taken together, this study offers a promising strategy for constructing a facile and safe PDA-based photothermal agent for photothermal antibacterial therapy.