The invention relates to a modular vehicle system, having an electric vehicle (2, 2′), in particular a lightweight electric vehicle, and at least one module (3, 120) which can be connected to the electric vehicle (2, 2′). The electric vehicle (2, 2′) has at least one on-board power supply system (13, 101) for supplying energy to an electrical drive unit (114), one or more interfaces (4, 114) which are connected to the on-board power supply system (13, 101) and are intended for connecting at least one module (3, 120), and a first locking means (9) which is arranged on the interface (4, 4′, 114). The at least one module (3, 120) has at least one electrical arrangement, a connecting element (6, 6′, 121) which can be connected disconnectably to the interface (4, 4′, 114) and has the purpose of connecting the electrical arrangement to the on-board power supply system (13, 101), and a second locking means (11) which is arranged on the connecting element (6, 6′, 121) and which is designed to engage with the first locking means (9). In order to provide a modular vehicle system (1) which has increased operational reliability and in which the connection to a module (3, 120) can be made in a particularly easy and secure fashion such that it is protected against intervention by unauthorized persons, at least one of the locking means (9, 11) can be moved between a free position and a locking position, wherein in the free position the connecting element (6, 6′, 121) can be disconnected from the interface (4, 4′, 114), and in the locked position the connecting element (6, 6′, 121) is mechanically locked to the interface (4, 4′, 114).