Novel functionalized and/or grafted crosslinked chitosan adsorbents were synthesized and used to remove several toxic heavy metal ions from contaminated water efficiently and selectively. The chitosan biopolymer was functionalized by maleic anhydride (CS_MA), which also acted as a crosslinking agent. Separately, Glutaraldehyde-crosslinked chitosan (CS_GA) was grafted with poly(methyl methacrylate) (CS_MMA). The synthesized adsorbents were used to adsorb and remove heavy metal ions from contaminated water. The metal ions were nickel, lead, chromium, and cadmium. The adsorption capacity of the adsorbents was analyzed under various conditions of contact time, adsorbent dose, initial concertation, temperature, and pH and evaluated against those of pure chitosan (CS) and the crosslinked one (CS_GA). The ultimate conditions for the ions removal were 0.5g/100ml adsorbent dose, an initial metal ion concentration of 50 ppm, a temperature of 45oC, and a pH 9. CS_MMA, which had the highest removal percentages for all metal ions, ranging from 92% to 94%. The adsorption was demonstrated to fit a pseudo-first-order model that followed a Langmuir adsorption isotherm. The results highlight the capacity of the synthesized polymers to efficiently remove major toxic contaminants at low cost from contaminated water, present especially in low-income areas, without harming the environment.