Folate deficiency, also known as vitamin B₉ deficiency, is a low level of folate and derivatives in the body. This may result in megaloblastic anemia in which red blood cells become abnormally large, and folate deficiency anemia is the term given for this medical condition. Signs of folate deficiency are often subtle. Symptoms may include fatigue, heart palpitations, shortness of breath, feeling faint, open sores on the tongue, loss of appetite, changes in the color of the skin or hair, irritability, and behavioral changes. Temporary reversible infertility may occur. Folate deficiency anemia during pregnancy may give rise to the birth of low weight birth premature infants and infants with neural tube defects.

Not consuming enough folate can lead to folate deficiency within a few months. Otherwise, causes may include increased needs as with pregnancy, and in those with shortened red blood cell lifespan. Folate deficiency can be secondary to vitamin B₁₂ deficiency or a defect in homocysteine methyl transferase that leads to a "folate trap" in which is an inactive metabolite that cannot be recovered. Diagnosis is typically confirmed by blood tests, including a complete blood count, and serum folate levels. Increased homocysteine levels may suggest deficiency state, but it is also affected by other factors. Vitamin B₁₂ deficiency must be ruled out, if left untreated, may cause irreversible neurological damage.

Treatment may include dietary changes and folic acid supplements. Dietary changes including eating foods high in folate such as, fruits and green leafy vegetables can help. Prevention is recommended for pregnant women or those who are planning a pregnancy.

Folate deficiency is very rare in countries with folic acid fortification programs. Worldwide prevalence of anemia due to folic acid deficiency generally is very low.

Signs of folate deficiency anemia most of the time are subtle. Anemia (macrocytic, megaloblastic anemia) can be a sign of advanced folate deficiency in adults. Folate deficiency anemia may result in feeling tired, weakness, changes to the color of the skin or hair, open sores on the mouth, shortness of breath, palpitations, lightheadedness, cold hands and feet, headaches, easy bleeding or bruising, low-grade fevers, loss of appetite, weight loss, diarrhea, decreased taste, irritability, and behavioral disorders.

Women with folate deficiency who become pregnant are more likely to give birth to low birth weight premature infants, and infants with neural tube defects and even spina bifida. In infants and children, folate deficiency can lead to failure to thrive or slow growth rate, diarrhea, oral ulcers, megaloblastic anemia, neurological deterioration. An abnormally small head, irritability, developmental delay, seizures, blindness and cerebellar ataxia can also be observed.

A deficiency of folate can occur when the body's need for folate is increased, when dietary intake or absorption of folate is inadequate, or when the body excretes (or loses) more folate than usual. Medications that interfere with the body's ability to use folate may also increase the need for this vitamin. Some research indicates that exposure to ultraviolet light, including the use of tanning beds, can lead to a folate deficiency. The deficiency is more common in pregnant women, infants, children, and adolescents. It may also be due to poor diet or a consequence of alcoholism.

Additionally, a defect in homocysteine methyltransferase or a deficiency of vitamin B₁₂ may lead to a so-called "methyl-trap" of tetrahydrofolate (THF), in which THF is converted to a reservoir of methyl-THF which thereafter has no way of being metabolized, and serves as a sink of THF that causes a subsequent deficiency in folate. Thus, a deficiency in B₁₂ can generate a large pool of methyl-THF that is unable to undergo reactions and will mimic folate deficiency.^{[citation needed]}