Ocean fertilization or ocean nourishment refers to both natural and intentional processes that replenish iron and other nutrients in the upper ocean, which in turn stimulate the growth of phytoplankton and in some circumstances draw down large amounts of carbon dioxide (CO₂) through photosynthesis. Intentional ocean fertilization is biomimicry of natural processes that have removed atmospheric CO₂ before ice ages as well as after volcanic eruptions, whale defecation, and near hydrothermal vents. The introduction of nutrients to the upper ocean increases marine food production as well as removing carbon dioxide from the atmosphere.

Ocean nutrient fertilization, for example iron fertilization, (OIF) can stimulate photosynthesis in phytoplankton. The phytoplankton converts the ocean's dissolved carbon dioxide into carbohydrate, some of which has been shown to sink into the deeper ocean. More than a dozen open-sea experiments confirmed that adding iron to the ocean increases photosynthesis in phytoplankton by up to 30 times.

Ocean iron fertilization is one of the more well-researched carbon dioxide removal (CDR) approaches, and supported by climate restoration proponents.^{[citation needed]} However, there is uncertainty about this approach regarding the duration of the effective oceanic carbon sequestration. A National Academies of Science, Engineering and Medicine (NASEM) 2021 study on marine CDR (mCDR) concludes that OIF has among the highest potential of mCDR approaches.

NASEM also calculates the cost of OIF at 40 cents per ton of CO₂ removed, although attendant research efforts would add additional cost. The report indicates that there is medium-high confidence that the technique could be efficient and scalable at low cost, with medium environmental risks. "This biotic approach has relatively high scalability and low costs for deployment, though challenges would include verifiable C accounting and, as for most ocean CDR at scale, careful monitoring of intended and unexpected ecological effects up and down the food chain."

Peter Fiekowsky and Carole Douglis write, "I consider iron fertilization an important item on our list of potential climate restoration solutions. Given the fact that iron fertilization is a natural process that has taken place on a massive scale for millions of years, it is likely that most of the side effects are familiar ones that pose no major threat."

A number of techniques, including fertilization by the micronutrient iron (called iron fertilization) or with nitrogen and phosphorus (both macronutrients), have been proposed. Some research in the early 2020s suggested that it could only permanently sequester a small amount of carbon. More recent research publications sustain that iron fertilization shows promise. A NOAA special report rated iron fertilization as having "a moderate potential for cost, scalability and how long carbon might be stored compared to other marine sequestration ideas"

The marine food chain is based on photosynthesis by marine phytoplankton that combine carbon with inorganic nutrients to produce organic matter. Production is limited by the availability of nutrients, most commonly nitrogen or iron. Numerous experiments have demonstrated how iron fertilization can increase phytoplankton productivity. Nitrogen is a limiting nutrient over much of the ocean and can be supplied from various sources, including fixation by cyanobacteria. Carbon-to-iron ratios in phytoplankton are much larger than carbon-to-nitrogen or carbon-to-phosphorus ratios, so iron has the highest potential for sequestration per unit mass added.

Oceanic carbon naturally cycles between the surface and the deep via two "pumps" of similar scale. The "solubility" pump is driven by ocean circulation and the solubility of CO₂ in seawater. The "biological" pump is driven by phytoplankton and subsequent settling of detrital particles or dispersion of dissolved organic carbon. The former has increased as a result of increasing atmospheric CO₂ concentration. This CO₂ sink is estimated to be approximately 2 GtC yr−1.