The realization of the topological field-effect transistor requires an electric field-induced transition from a topological state with dissipationless conductive channels (‘On') to a trivial insulator state (‘Off'). Monoelemental and buckled quantum spin Hall insulators with large topological band gaps are ideal candidates to investigate topological phase transitions. We provide compelling experimental evidence that low-buckled epitaxial germanene is a quantum spin Hall insulator with a bulk gap and robust metallic edge states. The low-buckled structure of germanene allows for topological phase transitions to take place. Upon the application of a critical transversal electric field, the topological gap closes and germanene becomes a Dirac semimetal. Increasing the electric field further results in the opening of a trivial gap and the disappearance of the edge states. This electric field-induced switching of the topological state and the sizeable gap make germanene suitable for room-temperature topological field-effect transistors, which could revolutionize low-energy electronics and spintronics.