<p>This study presents the development of a novel electrochemical sensor for the sensitive and selective detection of serotonin by a nanocomposite of chitosan and nanostarch. This environmentally friendly synthesized nanocomposite was characterized by using various techniques, including SEM, EDX, FTIR, XRD, UV–Vis spectroscopy, CV, and EIS. The sensor was fabricated by modifying a pencil graphite electrode with the synthesized nanocomposite. Electrochemical measurements of the electrode were performed using differential pulse voltammetry and square wave voltammetry. The sensor demonstrated excellent analytical performance, with a wide linear range, low limit of detection, high sensitivity, and selectivity for serotonin. The developed sensor showed detection limits of 1.41 ng/mL and 1.81 ng/mL, and quantification limits of 4.71 ng/mL and 6.04 ng/mL, with SWV and DPV, respectively. The practical applicability of the sensor was successfully demonstrated for the serotonin level in biological fluid samples. The proposed sensor offers a rapid, cost-effective, and sensitive approach for serotonin quantification, with potential for future applications in the simultaneous detection of multiple neurotransmitters.</p> Graphical Abstract <p></p>