Biosorption of nickel(II) ions from aqueous solution onto Sargassum wightii has been studied and the equilibrium isotherms were determined. The experimental data obtained at different pH conditions (pH 3.0–4.5) have been analyzed using five two-parameter models (Langmuir, Freundlich, Temkin, Dubinin–Radushkevich and Flory–Huggins) and five three-parameter models (Redlich–Peterson, Sips, Khan, Radke–Prausnitz and Toth). In order to determine the best fit isotherm, three error analysis methods were used to evaluate the data: correlation coefficient, residual root mean square error and chi-square test. The error analysis demonstrated that the three-parameter models better described the nickel biosorption data compared to two-parameter models. In particular, Toth equation provided the best model for nickel biosorption data at all pH conditions examined.