The hypothesis that the dried, ground biomass of the red marine alga Gracilaria tenuistipitata could be used for the efficient removal of lac dye from aqueous solution was assessed in this work. The effects of parameters such as initial pH, biosorbent dosage, contact time, initial dye concentration, and temperature on the biosorption capacity of the dye were investigated. Equilibrium data were analysed using Langmuir, Freundlich, and Temkin isotherm models, and the Freundlich model provided the highest coefficient of determination values. Biosorption kinetic data were successfully described with a pseudo-second-order model at initial dye concentrations of 50, 80, 100, and 120 mg l−1. The thermodynamic parameters of biosorption - enthalpy change (∆H° = −30.64 kJ mol−1), free energy change (∆G° = 4.32 kJ mol−1 at 303 K to 7.78 kJ mol−1 at 333 K), and entropy change (∆S° = −115.38 J mol−1 K−1) - were determined. The negative value of the enthalpy change and positive values of the free energy change indicate that the biosorption process is exothermic and non-spontaneous. The negative value of the entropy change is consistent with decreased randomness at the solid-liquid interface with dye biosorption. Attenuated total reflectance-Fourier transform infrared spectroscopic analysis confirmed the presence of lac dye on the G. tenuistipitata material. The efficiency of lac dye removal by this biomass material at 20 g l−1 and with an initial dye concentration of 50 mg l−1 in acidic solution was 71%, which indicated its potential usefulness as a new dye biosorbent.