In this study, a novel carbon paste potentiometric ion-selective electrode was developed and characterized for the determination of cyanocobalamin (vitamin B12). Its analytical performance and applicability in the determination of vitamin B12 in pure samples and pharmaceutical formulations were evaluated. The electrode was fabricated using an ion-pair complex formed by the reaction of cyanocobalamin with phosphomolybdic acid as the electroactive material. The ion pair was incorporated into a carbon paste consisting of graphite powder and a plasticizer. The effects of three plasticizers, namely dibutyl phthalate (DBP), dioctyl phthalate (DOP), and paraffin oil, on the potentiometric response of the electrode were investigated to obtain the optimum electrode composition. The analytical performance of the proposed electrode was evaluated using the direct potentiometric method by studying its Nernstian slope, linear concentration range, detection limit, response time, pH and temperature ranges, operational lifetime, and selectivity against interfering ions (vitamin B6, vitamin B1, Cu²⁺, K⁺, Na⁺, Zn²⁺, and Mg²⁺). The results showed that the optimum electrode composition contained 8% ion-pair complex with DBP as the plasticizer. The proposed electrode exhibited a Nernstian slope of 57.55 ± 0.16 mV/decade, a linear concentration range of (1.99 × 10⁻⁶–2.47 × 10⁻³) mol /L, and a detection limit of (1 × 10⁻⁶) mol/ L. The electrode showed the best performance within a pH range of (4–7) and a temperature range of (25–35) °C, with a response time 50 sec and an operational lifetime of about one month. The low selectivity coefficient values confirmed good selectivity against interfering ions. Statistical evaluation confirmed the accuracy and precision of the proposed method, with recoveries ranging from (95.8 - 105.6) % and low values of standard deviation (SD) and relative standard deviation (RSD). Furthermore, statistical comparison with the reference method using Student’s t-test and F-test revealed no significant differences between the two methods, confirming the suitability of the proposed electrode for the reliable and accurate determination of vitamin B12 in pure samples and pharmaceutical formulations.