<p>Multi walled carbon nanotube–chitosan (MWCNT–CHIT) nanocomposite modified disposable pencil graphite electrodes (PGEs) were developed for the electrochemical detection of miRNA-34a (miR-34a), a biomarker associated with Alzheimer’s disease and various types of cancer. Electrochemical measurements were carried out using a three-electrode system, in which the MWCNT–CHIT modified PGE served as the working electrode. The electrodes were first characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and microscopic techniques. An inosine-substituted miRNA-34a-specific DNA probe was used as the biorecognition element and hybridized with the target miRNA via complementary base pairing in solution phase. Following hybridization, the electrochemical detection was performed using differential pulse voltammetry (DPV) by monitoring the guanine oxidation signal. Due to the inosine substitution in the DNA probe sequence, the electrochemical response arises exclusively from the guanine bases of the target miRNA-34a. Under optimized conditions, the biosensor exhibited a linear detection range from 1 to 6&#xa0;µg/mL, with a detection limit of 0.569&#xa0;µg/mL (3.22 pmol in a 40 µL sample). The sensitivity was calculated as 1.52 µA µg mL⁻¹ cm⁻². Selectivity was evaluated using non-complementary miRNA sequences. Overall, this biosensor platform provides a rapid, cost-effective, and disposable biosensing system for miRNA-34a detection. The combination of an MWCNT–chitosan composite modified electrode with an optimized electrochemical detection strategy enables a simple and efficient analytical approach suitable for low-cost biosensing applications.</p>