Upwelling is an oceanographic phenomenon that occurs when strong, usually seasonal, winds push water away from the coast, bringing cold, nutrient-rich deep waters up to the surface.

Deep water nutrients consist of nutritional salts such as nitrates and phosphates, themselves the result of decomposition of the sea snow. When brought to the surface, these nutrients are processed by phytoplankton and combined with organic compounds such as dissolved CO2, using the energy of the sun through photosynthesis.

Upwellings therefore cause very high levels of productivity in phytoplankton compared to other areas of the ocean, and phytoplancton being the basic nutrient of most sea animals, these effects are propagated up the food chain.

Currently known regions of upwelling include coastal Peru, Arabian Sea, western South Africa, eastern New Zealand and the California coast, all very rich fishing grounds.

Localized upwelling may be due to deflection of deep currents by a seamount providing a nutrient rich island in otherwise low productivity ocean areas. These provide islands of life in such areas and are important to migrating species and human fishing. Upwelling can also occur when tropical cyclones stall over an area, moving at speeds of less than 5 mi/h (8 km/h). The churning of a cyclone eventually draws up cooler water from lower layers of the ocean. This causes the cyclone to weaken.

Upwellings also occur in other fluid environments, such as the magma in Earth's mantle or the plasma within a star. They are often a result of convection.