Material research has been a major driving force in the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices; identifying new magnetic materials is key to better device performance and new device paradigm. The advent of two-dimensional van der Waals crystals creates new possibilities. This family of materials retain their chemical stability and structural integrity down to monolayers and, being atomically thin, are readily tuned by various kinds of gate modulation. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr$_2$Ge$_2$Te$_6$ and CrI$_3$ at low temperatures. Here, we developed a new device fabrication technique, and successfully isolated monolayers from layered metallic magnet Fe$_3$GeTe$_2$ for magnetotransport study. We found that the itinerant ferromagnetism persists in Fe$_3$GeTe$_2$ down to monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, $T_c$, is suppressed in pristine Fe$_3$GeTe$_2$ thin flakes. An ionic gate, however, dramatically raises the $T_c$ up to room temperature, significantly higher than the bulk $T_c$ of 205 Kelvin. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe$_3$GeTe$_2$ opens up opportunities for potential voltage-controlled magnetoelectronics based on atomically thin van der Waals crystals.