The elevated concentration of copper (II) ions in wastewater has become a serious environmental and public health concern, highlighting the need for affordable and efficient treatment methods. In this study, modified guava leaf powder was utilized as a biosorbent for the removal of Cu²⁺ ions from synthetic wastewater. The guava leaves were treated sequentially with sodium hydroxide and tartaric acid to enhance their surface properties and improve adsorption performance. The prepared biosorbent was characterized using Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR), along with iodine number analysis to evaluate surface area and porosity. Batch adsorption experiments were conducted to assess the influence of operational variables, including pH levels (4, 7, and 10), contact time (30, 45, and 60 minutes), and biosorbent dosage (0.4, 0.7, and 1.0 g). Optimization of the adsorption process was performed using Response Surface Methodology (RSM) combined with a Box-Behnken Design. A quadratic model was developed to describe the relationship between the independent variables and Cu²⁺ removal efficiency, showing strong statistical validity and close agreement between predicted and experimental values. Optimal conditions were identified at pH 7, a contact time of 45 minutes, and a biosorbent dosage of 0.7 g, achieving a maximum copper removal efficiency of 97.122% with a desirability value of 1.000. Kinetic analysis indicated that the adsorption followed a pseudo-second-order model (R² = 0.9998), suggesting that chemisorption played a dominant role in the process. Isotherm studies further showed that the Freundlich model (R² = 0.5864) provided a better fit compared to the Langmuir model, implying that adsorption occurred on a heterogeneous surface. Overall, the results demonstrate that modified guava leaf powder is a promising, sustainable, and low-cost biosorbent for effective removal of copper ions from wastewater.