We have investigated the biosorption of pyrocatechol violet (PCV) from aqueous solutions by Robinia pseudoacacia tree leaves as a low-cost and eco-friendly biosorbent. A full factorial design was performed for screening the main variables and their interactions, which reduces the large total number of experiments. Results of the full factorial design (24) based on an analysis of variance (ANOVA) demonstrated that the initial PCV concentration, contact time, pH and temperature are statistically significant. Box-Behnken design, a response surface methodology, was used for further optimization of these selected factors. The ANOVA and some statistical tests such as lack-of-fit and coefficient of determination (R2) showed good fit of the experimental data to the second-order polynomial model. The Langmuir and Freundlich isotherm models were used to describe the equilibrium isotherms. Equilibrium data fitted well with the Freundlich isotherm model (R2 > 0.97). In addition, thermodynamic parameters (ΔG°, ΔH° and ΔS°) were calculated, these parameters show that the biosorption process was spontaneous (ΔG° = −2.423) and exothermic (ΔH° = −9.67). The biosorption kinetic data were fitted with the pseudo-second-order kinetic model (R2 > 0.999). These results confirm that R. pseudoacacia leaves have good potential for removal of PCV from aqueous solution.