<p>The presence of Cu²⁺ in water resources and the human body can be extremely harmful to health. The detection of Cu²⁺ in environmental liquid sources is very important in preventing copper-related diseases and environmental pollution. In this study, for the first time, a gallic acid- carbon paste electrode (GA-CPE) membrane potentiometric sensor was developed using GA acid by a simple method for detecting Cu²⁺ in various liquid samples. The structure of the GA-CPE membrane electrode of the sensor was elucidated by analyses of SEM–EDS. Using the GA-CPE sensor, potentiometric measurements of 14 different ions, including the Cu²⁺, were carried out in solutions (10^–2&#xa0;M), and their results were compared. The experimental results showed that the GA-CPE sensor exhibits selective potentiometric behavior toward Cu²⁺. The experimental results in 1.0 × 10^–2–1.0 × 10^–6&#xa0;M Cu²⁺ solutions revealed that the GA-CPE sensor has a 25.8 ± 1.12&#xa0;mV/decade Nernstian behavior and a linear curvilinear result (E = − 25.8 [− log(Cu²⁺)] + 1368.8 R² = 0.9996). The obtained GA-CPE membrane structured sensor was determined to be selective, stable, and highly sensitive in the detection of Cu⁺². The limit of detection of GA-CPE in the experiments performed with different concentrations of Cu²⁺ was found to be 1.92 × 10^–7&#xa0;M. DFT studies of the GA-CPE structure were performed to determine possible interactions and theoretical calculations. DFT studies of HOMO, LOMO, and band gap energy gaps were found to be − 6.4834&#xa0;eV, − 1.7015&#xa0;eV, and 4.7819&#xa0;eV, respectively. The experimental results were found to be in line with the theoretical studies. The experimental parameters using the GA-CPE sensor for detecting Cu²⁺, such as repeatability at a large number of concentrations, pH (5–11), and response time (7&#xa0;s), were successfully performed. As a result, our findings showed that the GA-CPE sensor is sensitive, low-cost, stable, and selective towards Cu²⁺.</p>