The Pierre Auger Observatory, located in Malargüe, Argentina, is the largest facility for the detection of ultra-high-energy cosmic rays and has been operating successfully for nearly 20 years. For its second phase of operation, the Observatory is undergoing a major upgrade, called AugerPrime, to increase its sensitivity to the primary mass. As part of the upgrade, the Underground Muon Detector is being deployed in the low-energy extension of the Surface Detector. It consists of an array of 30$\,$m$^2$ plastic scintillator muon counters buried 2.3$\,$m underground in the vicinity of the water-Cherenkov detectors. This will allow a direct measurement of the muonic component of air showers in the energy range $10^{16.5}$$\,$eV to $10^{19}$$\,$eV, contributing significantly to the discrimination of the primary mass and to the testing of hadronic interaction models. In this contribution, the deployment status and performance of the Underground Muon Detector are presented.