A creative combination of chitosan with polyacrylic acid (PAA) improves the acidity resistance of chitosan and increases its potential in the field of adsorption. In order to facilitate recovery, magnetic nanoparticles were incorporated in CS–PAA to obtain a magnetic-CS–PAA (MCS–PAA) nanocomposite. The physical and chemical characteristics of the composite adsorbent MCS–PAA were determined by SEM, TEM, FTIR, EDX, XRD, and XPS. This environmental-friendly, magnetic, composite adsorbent showed significantly better adsorption performance than those of the individual adsorbents alone. The maximal adsorption capacity was 204.89 mg/g according to the Langmuir isotherm model, when the concentration of Pb(II) was 100 mg/L at the equilibrium time of 70 min. The main adsorption mechanism was the complexation between the carboxyl, amino, and hydroxyl groups in MCS–PAA and Pb(II). Further, introduction of PAA also improved the acid resistance of CS. The new adsorbent MCS–PAA is thus expected to facilitate a wider range of applications for chitosan in the adsorption of Pb(II).