2D semiconducting materials have become the central component of various nanoelectronic devices and sensors. For sensors operating in liquid, it is crucial to efficiently block the electron transfer that occurs between the electrodes contacting the 2D material and the interfering redox species. This reduces current leakages and preserves a good signal-to-noise ratio. Here, a simple electrochemical method is presented for passivating the electrodes contacting a monolayer of MoS2, a representative of transition metal dichalcogenide semiconductors. The method is based on blocking the electrode surface by a thin and compact layer of electronically nonconductive poly(phenylene oxide), PPO, formed by electrochemical polymerization of phenol. Since the phenol polymerization occurs in the potential window where MoS2 is electrochemically inactive, the PPO deposition is area-selective, limited to the electrode surface. The deposited PPO film is characterized by electrochemical, X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy techniques. The applicability of this method is demonstrated by coating the electrodes of a MoS2-based field-effect transistor coupled with a nanopore. The highly selective deposition, the simple approach, and the compatibility with MoS2 makes this method a good strategy for efficient insulation of micro- and nanoelectrodes contacting 2D semiconductor-based devices.