Owing to its excellent biocompatibility, abundant functional groups and injectability, the gellan hydrogel is widely used in the biomedical field. In this work, a Au/MnO₂@GG nanocomposite hydrogel was prepared by a one-pot method using the gellan hydrogel as a template. The obtained Au/MnO₂@GG exhibits a broad plasmon-enhanced absorption in the near-infrared region. As a result, the nanocomposite hydrogel has an enhanced photothermal conversion efficiency of 71.3% at 808 nm, superior to those of MnO₂@GG (27.1%) and Au@GG (25.7%) alone. The local surface plasmon resonance induced by Au could transfer energy to MnO₂, and thus improve the photothermal conversion efficiency through non-radiative energy dissipation. Moreover, the nanocomposite hydrogel displays excellent biocompatibility, long-term stability, good shear-thinning and self-recovery properties. In vivo antitumor studies proved that the nanocomposite hydrogel could completely ablate the solid tumors under NIR illumination. This work may encourage the development of more advanced nanocomposite materials with high photothermal conversion efficiency, stability, and multiple functionalities.