Folate, also known as vitamin B₉ and folacin, is one of the B vitamins. Manufactured folic acid, which is converted into folate by the body, is used as a dietary supplement and in food fortification as it is more stable during processing and storage. Folate is required for the body to make DNA and RNA and metabolise amino acids necessary for cell division. As humans cannot make folate, it is required in the diet, making it an essential nutrient. It occurs naturally in many foods. The recommended adult daily intake of folate in the U.S. is 400 micrograms from foods or dietary supplements.

Folate in the form of folic acid is used to treat anemia caused by folate deficiency. Folic acid is also used as a supplement by women during pregnancy to reduce the risk of neural tube defects (NTDs) in the baby. Low levels in early pregnancy are believed to be the cause of more than half of babies born with NTDs. More than 80 countries use either mandatory or voluntary fortification of certain foods with folic acid as a measure to decrease the rate of NTDs. Long-term supplementation with relatively large amounts of folic acid is associated with a small reduction in the risk of stroke and an increased risk of prostate cancer. There are concerns that large amounts of supplemental folic acid can hide vitamin B₁₂ deficiency.

Not consuming enough folate can lead to folate deficiency. This may result in a type of anemia in which red blood cells become abnormally large. Symptoms may include feeling tired, heart palpitations, shortness of breath, open sores on the tongue, and changes in the color of the skin or hair. Folate deficiency in children may develop within a month of poor dietary intake. In adults, normal total body folate is between 10 and 30 mg with blood levels of greater than 7 nmol/L (3 ng/mL).

Folate was discovered between 1931 and 1943. It is on the World Health Organization's List of Essential Medicines. In 2020, it was the 67th most commonly prescribed medication in the United States, with more than 10 million prescriptions. The term "folic" is from the Latin word folium (which means leaf) because it was found in dark-green leafy vegetables.