Alfalfa (Medicago sativa) is a flowering plant in the pea family Fabaceae cultivated as an important forage crop. In the UK, Australia, and New Zealand it is known as lucerne and as lucerne grass in south Asia.

Alfalfa is a cool season perennial legume living from three to twelve years, depending on variety and climate. It resembles clover with clusters of small purple flowers. The plant grows to a height of up to 1 metre (3 ft), and has a deep root system sometimes stretching to 4.5 metres (15 ft). This makes it very resilient, especially to droughts. It has a tetraploid genome. The plant exhibits autotoxicity, which means that it is difficult for alfalfa seed to grow in existing stands of alfalfa. Therefore, it is recommended that alfalfa fields be rotated with other species (for example, corn or wheat) before reseeding.

Like other legumes its root nodules contain bacteria, Sinorhizobium meliloti, with the ability to fix nitrogen, producing a high-protein feed regardless of available nitrogen in the soil. Its nitrogen-fixing abilities (which increases soil nitrogen) and its use as an animal feed greatly improved agricultural efficiency. (The nitrogen comes from the air, which is 78 percent molecular nitrogen.)

Alfalfa is widely grown throughout the world as forage for cattle, and is most often harvested as hay, but can also be made into silage, grazed, or fed as greenchop. Alfalfa has the highest feeding value of all common hay crops, being used less frequently as pasture. When grown on soils where it is well-adapted, alfalfa is the highest yielding forage plant.

Alfalfa is one of the most important legumes used in agriculture. The US is the largest alfalfa producer in the world, but considerable area is found in Argentina (primarily grazed), Australia, South Africa, and the Middle East. Known as Kuthirai Masal in Tamil, alfalfa is mostly grown in the Coimbatore district of Tamil Nadu, southern India.

Within the U.S.A. the leading alfalfa growing states are California, South Dakota, and Wisconsin. The upper Midwestern states account for about 50% of US production, the Northeastern states 10%, the Western states 40% and the Southeastern states almost none. Alfalfa has a wide range of adaptation and can be grown from very cold northern plains to high mountain valleys, from rich temperate agricultural regions to Mediterranean climates and searing hot deserts.

Its primary use is as feed for dairy cattle—because of its high protein content and highly digestible fiber—and secondarily for beef cattle, horses, sheep, and goats. Humans also eat alfalfa sprouts in salads and sandwiches. Tender shoots are eaten in some places as a leaf vegetable. Human consumption of fresh mature plant parts is rare and limited primarily by alfalfa's high fiber content. Dehydrated alfalfa leaf is commercially available as a dietary supplement in several forms, such as tablets, powders and tea. Alfalfa is believed by some to be a galactagogue, a substance that induces lactation.