Two-dimensional materials have attracted a great deal of interest in developing nanodevices for gas-sensing applications over the years. The 2D BeN₄ monolayer, a recently synthesized singlelayered Dirac semimetal, has the potential to function as a gas sensor. This study analyzes the NH₃ sensing capacity of the pristine and vacancy-induced BeN₄ monolayers using firstprinciples density functional theory (DFT) calculations. As per the results, the NH₃ molecule is physisorbed on the pristine BeN₄ via weak Van der Waals interaction with a poor adsorption energy of −0.41 eV and negligible charge transfer. Introducing Be vacancy in BeN₄ increased the NH₃ adsorption energy to −0.83 eV due to the improved charge transfer (0.044 e) from the defective monolayer to the NH₃ molecule. The structural stability, sufficient recovery time (74 s) at room temperature, and superior work function sensitivity promise the potential application of defective BeN₄ as an NH₃ sensor. This research will be a theoretical groundwork for creating innovative BeN₄-based NH₃ gas sensors.