•Breakthrough: Demonstration of fg/mL detection of AB42 peptides using electrospun SnO2 nanofibers (SNF).•Validation of detection methodology using real-time pooled human PLASMA samples.•Facile, sensitive, and label-free detection methodology with a wide range of detection ranging from 1fg/mL to 1ng/mL.•Superior performance in terms of stability, selectivity, repeatability, and reproducibility.•Precise detection of AB42 peptides in the presence of interfering compounds through effective reduction of matrix effect. Uncontrolled fibrous aggregation of proteins in the human brain implicates a range of anomalous biological phenomena, ultimately leading to Alzheimer's Disease (AD). Aggregates of β-Amyloid(1-42) (AB42) have been considered the most viable biomarker for early diagnosis of AD; therefore, it is highly essential to detect AB42 peptides in easily accessible body fluids, preferably at low concentrations. Considering this, we report the design and development of a facile, sensitive, and label-free electrochemical biosensor for AB42 peptide detection, using electrospun SnO2 nanofibers (SNF) as the transducing material. The sensing platform, comprising of AB42-specific capture antibodies covalently immobilized onto SNF nanofiber modified carbon working electrodes, acts as an immunoassay on to which the target analytes bind specifically. In response, the charge transfer resistance at the sensor interface gets modified proportionately and is recorded using electrochemical impedance spectroscopy. Herein, we have tested the efficiency of the said immunoassay against AB42 spiked buffer and plasma samples, in the concentration range 1 fg/mL–1 μg/mL. The proposed platform accounts for a sensitivity (limit of detection (LoD)) of 274.96(kΩ/ng.mL−1)/cm2 (0.146 fg/mL) and 302.05(kΩ/ng.mL−1)/cm2 (0.638 fg/mL) for AB42 spiked buffer and plasma samples, respectively. Furthermore, the proposed SNF-derived electrochemical immunoassay shows appreciable stability (over 126 days), selectivity, repeatability, reproducibility, and interference-resistance.