The importance of early detection of neurodegenerative disorder biomarkers has grown since these biomarkers are essential for timely diagnosis, treatment, healthcare, and wellness applications. We present a cost-effective and disposable electrochemical immunosensing strip for rapid, decentralized detection of brain-derived neurotrophic factor (BDNF)one of the major neurotrophins (NTs) associated with neurological and psychiatric disordersin human saliva. The salivary BDNF immunosensor strip is made on a screen-printed carbon electrode functionalized with carbon spherical shells (CSSs), polyethylenimine (PEI), and glutaraldehyde to enhance sensitivity. Through systematic optimization, the sensor achieved excellent analytical performance, with a wide dynamic detection range from 1.0 × 10 to 1.0 × 10 g mL, a rapid response time of under 3 min, and an ultralow detection limit of 1.0 × 10 g mL for BDNF in human saliva. The BDNF immunosensor demonstrated high selectivity, reproducibility, robustness, stability, and long-term storage capability. At a cost of less than US$ 2.19 per unit, this disposable sensor also enabled rapid BDNF detection in saliva samples collected from healthy volunteers without interference from other salivary constituents. The environmental impact was assessed using the Analytical Eco-Scale (AES), the Analytical GREEnness Metric Approach (AGREE), and the Blue Applicability Grade Index (BAGI), which evaluates the practicality ("blueness") of the device. These assessments confirmed the sustainability of the disposable BDNF immunosensor strip. This device provides a rapid, efficient, cost-effective, and reliable method for the decentralized, noninvasive salivary analysis of BDNF, enabling broader applications in healthcare, wellness monitoring, and medical diagnostics related to the neurotrophin family of biomarkers.