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Making use of the osmotic pressure difference between fresh water and seawater is an attractive, renewable and clean way to generate power and is known as ‘blue energy’^,,. Another electrokinetic phenomenon, called the streaming potential, occurs when an electrolyte is driven through narrow pores either by a pressure gradient or by an osmotic potential resulting from a salt concentration gradient. For this task, membranes made of two-dimensional materials are expected to be the most efficient, because water transport through a membrane scales inversely with membrane thickness^,,. Here we demonstrate the use of single-layer molybdenum disulfide (MoS₂) nanopores as osmotic nanopower generators. We observe a large, osmotically induced current produced from a salt gradient with an estimated power density of up to 10⁶ watts per square metre—a current that can be attributed mainly to the atomically thin …