Developing low-cost and efficient materials for dye pollutant removal under mild condition remains a great challenge. Here K+ and Tb3+ co-doped porous MnO2 (K–Tb–MnO2) nanoparticles with tailored properties including crystal structure, surface area and catalytic activity have been synthesized. Experimental results reveal that K–Tb–MnO2 nanoparticle has higher specific surface area, Mn3+ content and surface oxygen vacancies than pristine MnO2 nanoparticle and single-doped MnO2 materials, showing the uniqueness of dual-doped metal ions. Using methyl blue (MB) as a model pollutant, its removal efficiency by K–Tb–MnO2 nanoparticles within 5 min is 93.6%, which is 18, 8.3, and 2.9 times higher than that of MnO2, K–MnO2, and Tb–MnO2 nanomaterials, respectively. Oxalic acid triggered MnO2 material dissolving assay and FT-IR spectrum suggested that remarkable performance of K–Tb–MnO2 nanoparticle toward MB removal can be attributed to a combined effect of adsorption (16% MB removal) and catalytic degradation (84% MB removal). Moreover, K–Tb–MnO2 nanoparticle mediated MB degradation is demonstrated to be a combination of non-radical oxidation by Mn3+ and radical-participated degradation, with 1O2 as the main species. And the intermediates and pathways of MB degradation were studied by liquid chromatography-mass spectrometry. Importantly, cell viability experiment suggests that the toxicity of MB dye could be efficiently alleviated after the treatment with K–Tb–MnO2 nanoparticle. These results demonstrate the great potential of the novel K–Tb–MnO2 particles to be used as a highly effective nanomaterials to reduce the risk of dye wastes toward the environment and human health.