Early diagnosis of neurodegenerative diseases (NDs) such as Alzheimer’s (AD) and Parkinson’s (PD) is critically dependent on detecting specific protein biomarkers at very low concentrations. This study presents the design, fabrication, and characterization of a novel label-free electrochemical biosensor for ultrasensitive detection of key ND biomarkers (β-amyloid (Aβ), tau, and α-synuclein). A glassy carbon electrode (GCE) was modified with a nanocomposite of carboxyl graphene (CG), thionin, and electrodeposited Au nanoparticles (AuNPs) to immobilize a specific aptamer against tau protein. Electrochemical techniques (cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)) were used to verify each modification step, and differential pulse voltammetry (DPV) was used to measure biomarker binding. The sensor yielded a linear response from 0.5 pM to 100 pM tau₃₈₁, with a detection limit (LOD) of ~0.5 pM and high sensitivity. These performance metrics are comparable to or exceed those of previously reported label-free biosensors. Cross-reactivity tests showed excellent selectivity against non-target proteins. The fabricated biosensor demonstrates rapid, reproducible detection of tau at clinically relevant levels, addressing key research questions: (1) Can a label-free electrochemical biosensor be engineered to detect neurodegenerative biomarkers at ultra-low concentrations? (2) What sensitivity and specificity can be achieved through nanomaterial-enhanced signal transduction? (3) How does this sensor performance compare with existing biosensing platforms? In answering these questions, this work contributes to the development of point-of-care ND diagnostics.