Actuators that convert electrical energy to mechanical energy are useful in a wide variety of electromechanical systems and in robotics(1-6), with applications such as steerable catheters(7), adaptive wings for aircraft and drag-reducing wind turbines(8). Actuation systems can be based on various stimuli, such as heat, solvent adsorption/desorption(4,9), or electrochemical action (in systems such as carbon nanotube electrodes(1,10), graphite electrodes(11), polymer electrodes(6,12-14) and metals(15)). Here we demonstrate that the dynamic expansion and contraction of electrode films formed by restacking chemically exfoliated nanosheets of two-dimensional metallic molybdenum disulfide (MoS2) on thin plastic substrates can generate substantial mechanical forces. These films are capable of lifting masses that are more than 150 times that of the electrode over several millimetres and for hundreds of cycles. Specifically, the MoS2 films are able to generate mechanical stresses of about 17 megapascals-higher than mammalian muscle (about 0.3 megapascals) 3 and comparable to ceramic piezoelectric actuators (about 40 megapascals)-and strains of about 0.6 per cent, operating at frequencies up to 1 hertz. The actuation performance is attributed to the high electrical conductivity of the metallic 1T phase of MoS2 nanosheets, the elastic modulus of restacked MoS2 layers (2 to 4 gigapascals) and fast proton diffusion between the nanosheets. These results could lead to new electrochemical actuators for high-strain and high-frequency applications.