In this study, all-solid-state type potentiometric PVC membrane selective microsensor was developed for Metformin (MET) which is an antidiabetic drug active substance. Metformin-tetraphenylborate (MET-TPB) ion-pair was used as an ionophore in the structure of the sensor membrane. It was determined that the sensor membrane at the ratio of 69 % o-nitrophenyl octyl ether, 27 % polyvinyl chloride and 4 % MET-TPB performed the best potentiometric performance. In a wide concentration range (1x10(-5)-1x10(-1) mol/L), the slope, detection limit, response time, pH range, and life-time of the sensor were determined as 55.9 +/- 1.6 mV (R-2=0.996), 3.35x10(-6) mol/L, 8-10 s, pH: 3-8, and similar to 10 weeks, respectively. The voltammetric performances of the sensor were also investigated. The prepared microsensor was successfully utilized for the determination of Metformin in a pharmaceutical drug sample by potentiometry and voltammetry. It was observed that the obtained results were in agreement with the results obtained by the UV spectroscopy method at 95 % confidence level.