Hyperhomocysteinemia or hyperhomocysteinaemia is a medical condition characterized by an abnormally high level of homocysteine in the blood, conventionally described as above 15 µmol/L.

As a consequence of the biochemical reactions in which homocysteine is involved, deficiencies of vitamin B₆, folic acid (vitamin B₉), and vitamin B₁₂ can lead to high homocysteine levels.